[Windows Phone 7 Development and Hacking] GoodDayToDie:
First of all, the OS version does *not* matter here, it's all about the firmware version or bootloader version.

Second, "jailbreak" is a very ambiguous term on WP7, and one I really wish people wouldn't use, because there are multiple levels of unlocking.

The first, developer-unlock, is actually a feature of the OS and is supported by Microsoft. All phones can be dev-unlocked, but normally you have to pay for it (an AppHub account, intended for Marketplace developers, lets you unlock up to 3 phones and install up to 10 unsigned apps on each, for $99/year). Students can get a free account through DreamSpark, but only for one phone and up to three unsigned apps at once. There's another way, for $9 a ChevronWP7 Labs token lets you unlock one phone and put up to 10 unsigned apps on it. Unfortunately, those tokens are sold out and we have heard nothing on them getting any more (had to be approved by Microsoft). There are also some hacks that will dev-unlock a phone, such as WindowBreak (this thread). There aren't currently any such hacks available for HTC, though.

Next, there's interop-unlock. This means removing the 3- or 10-app limit, and also allowing the installation of apps that can use system components (drivers and long-running services) to break out of their sandboxes. These apps, commonly called "interop" apps after the ID_CAP_INTEROPSERVICES line in their manifests' that enables this capability, include things like registry editors and the HtcRoot project. All ways of getting interop-unlock are based on hacks. For Samsung phones, WindowBreak can be used to interop-unlock at the same time as dev-unlock. For HTC phones, though, you need to first be dev-unlocked before you can interop-unlock.

The third level is called "full unlock" and means the policy system that controls app sandboxing and user permissions has been disabled completely. Any app can run, and all apps run with full access to the whole device. Although this heightens the risk of malware or o accidentally damaging the device, it also allows all kinds of cool features to be added right into the phone, as well as enabling some very cool apps (including ports of WinMo apps). Currently, the only way to get full-unlock is to use a custom ROM.

In order to use a custom ROM, your device must have an unlocked bootloader. This is done by installing an alternate bootloader called RSPL, and can be made permanent by installing HSPL. You can read the DFT (Dark Forces Team, who are responsible, directly or indirectly, for almost all custom ROM achievements on WP7) threads on R/HSPL on this forum. Although all first-gen HTC phones are now supported for xSPL, be aware that it can only be installed over stock SPL (bootloader) versions on 3.1 or lower. Most fully up-to-date phones will be running SPL 5.x.

For you, in particular, you can either try interop-unlocking, or you can try installing an unlocked bootloader and a custom ROM. The first option preserves your phone data, but is less permissive in what you can do. The second option allows almost anything, but you'll have to wipe the phone and will also be at the mercy of any bugs in the custom ROMs. Additionally, either option may require rolling back the phone in some way (firmware for interop-unlock, SPL for bootloader unlock), and interop-unlock will require first getting dev-unlock (may cost money).