Hate to tell you, but T-Mobile has nothing that matches your requests. Their WinMo Pro phone, the Wing, is slow and lacks GPS.
AT&T has a better selection, though nothing slim and light that also has all those features and runs the Pro version of WinMo. There are recently released unlocked GSM phones that do match-- the HTC Touch Diamond seems like a perfect match and it's even very slim and light. Mom will like the voice quality-- my mom does . Here's our review: http://www.mobiletechreview.com/phones/HTC-Touch-Diamond.htm . It sells for around $650 though-- not cheap. You might want to take a subsidized phone on a new contract and sell it to help partially pay for the Diamond.
A very recent phone is the Samsung Omnia 900 (just came out a few weeks ago). It's another good match, though not as small as the Diamond. However, it's even harder to find and more expensive.