The T-Mobile MDA is just a re-branded HTC Wizard so look for screen protectors for that model.
My personal preference for screen protectors is for the WriteShield screen protectors over at
Pocket PC Techs.
I have never heard of the problem that you describe and would guess that it is the result of how the anti-glare pattern on the screen protector lines up with the pixel matrix on the screen. Makes me wonder if you would run into the same issue with other screen protectors.
You might want to avoid the same model screen protector you had for your Palm or maybe try one of the "clear/glossy" type protectors (or if your Palm screen protector was a clear/glossy type, try an anti-glare).