Not true, it's mostly open source. Conveniently [a bit of sarcasm there], a lot of the common setbacks of the OS are the closed source components such as the video driver.

Anyway, if you're looking for a phone, don't get the N900. Although it's a great device that can make calls and texts, it's not a great phone. You're better off with a higher-end Android if you're looking at OS-hacking and customizing while still having a decent phone. Android may not be true-linux, but many of the popular Maemo mods, apps, and functions are also available for Android, many of them work better, for instance, Gameboid is a GBA emulator that runs at 100% speed for just about everything, the UI is graphically accelerated on the better ones, Maemo's is not, root access is available on many devices, and although it's not as easy to set up, it gives you just as much power. I ordered an HTC Vision that I'll be receiving tomorrow to use alongside the N900, and I'm quite impressed by what it can do, me being an N900 diehard.

There's a pretty good, non-fanboy opinion.