Hmmm,

as you can see, people are working on various apps for N900. But even, when they will be available, you will not have PROBABLY some features like MMS (maybe you're using them often now...). The truth is that N900 is "step 4 of 5" and you can have some issues at least in next few months.

From the other hand: Win Mobile 6.5 (HTC HD2) is still Win Mobile with some disadvantages (Win Mobile 7 is expected to be released in the end of 2010).

Maybe you should think about some Android device then ? There are few nice models already and HTC should have few killer models in next few months All very Google related.

If not, you have only iPhone with own issues (closed OS or breaking it with each next release) or maybe Omnia HD/N97 with Symbian.

I'm afraid, that you should check each one and select device, which is the best in YOUR opinion. Or wait at least few months