In case someone still had doubts that it was safe to use the N900's LED flash as a light source, I have a quick story.

A few days ago, my dorm's bathroom lights burned out. Maintenance hasn't fixed it yet. When using the bathroom, or showering, I have to use the N900's LED flashlight. It provides enough light that if pointed at a wall, the entire bathroom is visibly illuminated.

Better yet, today my GF and I showered together while only having the N900's LED's to illuminate the bathroom (I left it out of reach of water, obviously). For at least 15 minutes (prob'ly longer), the LEDs were constantly on using the flashlight status menu applet. They still work just fine, didn't burn out, noticeably change or heat up, etc. So while I realize hardware may have some differences, I suspect that you can file this away as yet another reason to think the flash LEDs are perfectly safe to use as a flashlight, even for longer periods of time.