[Touch Diamond, MDA Compact IV General] tomchap:
Mmm...

Well i started a new Contract with tmobile (already have an exisiting one, but impatient waiting for the Diamond!) and I used to have the Web and Walk plus added to my account.

However, ive noticed that I get HSDPA on my new phone using the standard web n walk £7.50 package.

This is also what I found out when looking at the tmobile website:

http://www.t-mobile.co.uk/services/mobile-internet/on-your-phone/

Scroll to the bottom and then click on

Web n Walk Legal Stuff.

For the £7.50 a month Web n Walk Package

"Fair use policy applies: Subject to coverage. Compatible handset required. Roaming charges apply. Not applicable to connection via CSD. *To ensure a high quality of service for all our customers a 1GB (of data both sent and received in the UK) per month fair use policy applies. If you exceed 1GB per month, we may tell you to reduce your future use. If you again exceed 1GB per month, we may tell you that we are going to apply network protection controls, resulting in a reduced network speed for 14 days. If you exceed 1GB per month for a third time , we may tell you that we are going to apply further network protection controls, resulting in a reduced network speed until you upgrade your price plan to include a larger data allowance. We do not permit use of this service to provide modem access for a computer or for peer to peer file sharing, internet phone calls or instant messaging. Roaming charges apply."

and for the Plus package

"If your pay monthly plan already includes web'nwalk, Mobile Broadband Plus will be £5.00 extra a month and if you don't, it will be £12.50 extra a month. Mobile Broadband Plus will stay on your account until you ask us to remove it. You'll need a compatible laptop, compatible phone and mobile broadband coverage. Remember that you can't use your plan to make internet phone calls. If you use your plan outside of the UK, we'll add roaming charges to your pay monthly account, please see t-mobile.co.uk//broadband for more details. This plan comes with a fair use policy of 3GB a month. We'll monitor how much you send and receive each calendar month so that we can protect our network for all our Mobile Broadband customers. If you use more than your fair use policy amount, we won't charge you any more, but we may restrict how you can use your plan, depending on how often you go over your amount and by how much."

and for the Max Package

"Mobile Broadband Max is £22.50 extra a month and is for use in the UK only. You'll need a compatible laptop, a compatible phone and mobile broadband coverage. If you use your plan outside of the UK, we'll add roaming charges to your pay monthly account. This plan comes with a fair use policy of 10GB a month. We'll monitor how much you send and receive each calendar month so that we can protect our network for all our Mobile Broadband customers. If you use more than your fair use policy amount, we won't charge you any more, but we may restrict how you can use your plan, depending on how often you go over your amount and by how much."

Now i dont see anywhere suggesting that the speed increases with the more you spend, but the Data Allowance and restrictions eg Using Instant Messaging and VOIP are added on with the more you pay.

Hope this is of some use :)