I have an idea....

It is due to paid/protected apps not being available (DRM crap!).
This also happens when you root legit android phones. Happened on my X10.
Solution: I ran a fix via ABD for the x10, it MAY work on the N900?
It's designed for X10 and 1.6 Donut, not Froyo.
I will try it when I get time, I don't mind reflashing.
It's a 24 hours turnaround before you know if it works.

Essentially, it's a matter of modifying your "fingerprint" on Google servers, and when they do their refresh/reboot phase every 24hours, you can then have all apps available!

it is based on: XDA X10 Android Market Fix
I have the original files on my computer somewhere and I'll see what can be done or what may need to be modified to fix it to work with Froyo (or pass it onto e-yes and DJ_Steve).