Scratchbox is a development environment that allows to you cross-compile programs for armel-based processors (in this case, the one for n900) on x86 based platforms.

Using it and Xephyr, you can test (with varying degress of success) programs for your NIT. While it's not an emulator per se, it does allow you to see how your program would look. I've attached an images showing it running under KDE 4.3 in Linux. My desktop is rather large for a screenshot, so I only captured the window itself.

You can find more [accurate] information my clicking on the Development link above and selecting "Download the Maemo 5 SDK".

Anyways, my N900 comes today, so I'll be able to see what's going on.