At this point I am wondering if there is something that the community cannot do with the N900? The latest in the line of hacks to come out for the N900 is USB Host Mode. The same functionality that will out of the box on the Nokia N8, can greet your N900 as well if you are happy to hack around quite a bit.
As you can see from the following pictures, connecting a USB flash drive is a go and it pops up in the File Manager once connected.
If you would like more than one flash drive connected, that is a possibility too.
At this point it is not available for the average consumer to try, but if modding is your thing, here is the garage entry for it. If you would like to follow this development more closely, this is the thread to keep an eye on.