Here's a thought; is it possible to get skype or googletalk to work on the main camera, either natively or via Easy Debian? If so, an N900 could give any TV with a composite or SCART input smart-TV like video calling capability.

EDIT: It also gives the opportunity to play media off other UPNP devices on a not-so-smart TV.

Well, a bit off-topic, but still. About 4 years ago, when tablets were just entering the market, I got various chinese ones just to resell. Among those, there was one of particular interest for me back then. It was 7" resistive screen (800x480) Android tablet with the following hardware:
TI OMAP 3530 Cortex A8 clocked at 600MHz
RAM of 256 MB
WiFi 802.11b/g
MicroSD slot
USB OTG port
GPS
3000 mAh removable battery
Stereo speakers
Stylus, kickstand
Sounds familiar, doesn't it? Yeah, I wondered back then of the posibility to port Maemo to such a similar hardware. That would be the groundbreaker, had it actually become reality. Eventually, I sold it very quickly, so I had not had much time to play with that thing. I'll try to find some more info, maybe photos. That could be the first of Maemo-powered tablets, had Steven Flop not killed the Meego-Maemo lineage.

Another thought, gleaned from other threads and blogs: a permanently plugged in N900 as a security camera, either permanently recording (starting a new file each hour, for example, and deleting the oldest one after a predefined period) or on some sort of trigger, either when a sound threshold is reached (a la Babyphone) and/or when the camera's input changes (eg a significant difference between 2 frames; I have no idea how difficult that would be to code.)

This last is where it gets more fun. The device's wi-fi is of course on - what, you thoght storing security footage on the device only, which an intruder could nick, was a good plan? That's what the cloud's for. On trigger, the device could send a message via any one of a plethora of protocols, alerting you. Could we stream the video across the net to see what's happening? Of course, the low-light video mode's not much cop, so why not have the trigger also have the device tell wi-fi controlled lights to come on, both to give a better view and to startle any intruder?

It appears someone has not only thought of that, but already written the software: http://talk.maemo.org/showthread.php?t=86285