[G2 and Desire Z General] kusuriurikun:
With a different user agent, it will optimise pages for mobile viewing--but IN PRACTICE this mostly means that unnecessary crap like Flash gets stripped (Javascript remains OK).

One possible reason Safari might not be targeted--even with the "Mozilla/5.0" ID (which is so common even varieties of IE use it!), practically all mobile browsers identify as some variety of Safari or Safari-derivative (the iPhone notably so, but this also includes Chrome, the Android default browser, and Dolphin HD). It is entirely possible (depending on how smart or how dumb the sniffing T-Mobile does on HTTP GET requests is) that the software T-Mobile uses flags all "Safari"-labeled browsers as mobile browsers, possibly including Safari proper. (This isn't too far-fetched, as Mobile Safari is closer to Safari proper than (say) Pocket IE is to Internet Explorer or the Google default browser is to Chrome.)

A very interesting experiment, IMHO, to try would be to see if Opera Mobile is blocked as "tethering" and if it's possible to surf with the Windows version of Opera in tethering; Opera Mobile is (to my knowledge) the sole Android-based browser that does NOT identify as a Safari derivative. (Its User Agent, based on my G2 running ILWT CM7 185, is "Opera/9.80 (Android 2.3.5; Linux; Opera Mobi/ADR-1107051709; U; en) Presto/2.8.149 Version/11.10"; Opera 11.51 on WinXP identifies as "Opera/9.80 (Windows NT 5.1; U; en) Presto/2.9.168 Version/11.51".

If Opera Mobile is blocked, it's a good indicator they're sniffing for the presence of "Safari" in the User-Agent somewhere--especially since Safari proper apparently makes it through.

If the "Desktop" mode of Dolphin HD makes it through, this pretty much proves they're filtering for "Safari"--it identifies as Safari for MacOS X Snow Leopard, as we'll see below. (This also makes workarounds MUCH easier.)

(One thing that would be helpful in this regard--hit http://whatsmyuseragent.com using Safari and cut and paste the User-Agent displayed. This would be VERY useful for comparison with existing iPhone and Android User-Agents, and could even be potentially used to make a custom User-Agent that mollifies T-Mobile but lets non-mobile content through.)

As you noted below :D, it should be possible--I'd probably suggest (since we know that Safari for Windows works) copying the User-Agent string from Windows Safari. (In fact, Dolphin's "Desktop" mode exactly clones the User-Agent from Safari for MacOS X "Snow Leopard" (10.6.x).)

I'd say probably the ONE major kvetch I have about Chrome (as a browser) is that its APIs are very locked down--yes, I know, for purposes of security and all--but in a way that makes it less customisable where I'd LIKE it to be customisable :D

(A good example is how AdBlock Plus works on Firefox versus how AdBlock Plus works on Chrome. On Firefox, the extension works much more like a browser-specific host file or proxy, and prevents the adverts from even loading in the first place; Chrome's API doesn't allow this, and has to block the adverts AFTER loading them (basically comparing downloaded URLs to an internal blacklist). Not usually an issue with mere ad blocking, but if you use ad blocking software to prevent tracking (which I do) or to block known malware sites (which I do--I don't use the browser for pr0n or Illegal Activities but there have been notable cases where banner adverts on reputable sites have been compromised to send malware)...blocking after the fact is not as useful, alas.)

Probably the best way (if you're on Windows) to implement this sort of thing on Chrome is to copy your existing Chrome shortcut, rename it "Tethering Chrome", right-click and select "Properties", and add --user-agent="<user agent>" (with the stuff in brackets being whatever User Agent you want to use) to the "Target" section.

For example, the "Target" of my "tethering Chrome" shortcut would read something like "C:\Program Files\Google\Chrome\Application\chrome.exe" --useragent="Mozilla/5.0 (Macintosh; U; Intel MacOS X 10_6_3;en-us) AppleWebKit/155.16 (KHTML, like Gecko) Version/5.0 Safari/533.16" (the latter is the "Desktop" default User-Agent for Dolphin HD--which is also the EXACT User-Agent for the version of Safari shipping with MacOS X Snow Leopard).

As an aside, you can do this similarly in Linux, and PRESUMABLY in MacOS X (I don't muck with the latter too much, don't quote me on that).

Glad to be useful now and again :D

Good on you to point that out (and that's part of why I didn't recommend extensions for Chrome--it's a wonderful browser that "just works", but its extensions are EXTREMELY sandboxed--for safety, but sometimes this breaks stuff like network-level user detection, preemptive ad-blocking (preventing the ads from downloading before being redacted from a website), etc. Different approaches between Chrome and Firefox as to how their extensions work--Firefox extensions will let you muck with the inner workings of the browser itself, but occasionally to one's detriment :D; Chrome prefers sandboxed extensions which run safely but can't get "browser kernel access" when it needs it.

(A good parallel to how these work that is Very Relevant To Users' Interests here: Firefox's extension management works like a rooted phone, Chrome's like an unrooted stock G2 where system apps and network stuff are protected.)

The workaround (setting up an alternate shortcut to Chrome with an explicit user-agent defined on the command line) DOES seem to get around network detection (for things like testing browser functionality).