Once USB host is fully sussed out and available to end users, the same two potential barriers will apply every possible USB device:

• power draw
• driver availability

The power problem can be worked around via a powered USB hub. That effectively converts the power problem into two sub-issues:

• must stay near a power outlet
• must carry a larger amount of gear

The first of those may be avoided via the purchase of a rare, but available, battery-powered USB hub. The second is only as limiting as you let it be:
In the end, the availability of drivers becomes the real issue. For any given device, search to see if Linux drivers are available for it. If so, there's a good chance it may be made to work with the N900. If you're not into compiling software and fixing "works on x86 but breaks on ARMEL" bugs, then you'll need to woo a friendly Linux geek.