Only some stupid person would do it! (Not saying you are stupid... But if you do it, you actually are)
It's like the other tricks I don't like... Running Windows and Mac and Android... Well, Android is at least a bit better than the other two!

But seriously, what are you thinking about? You've bought a device, with maybe the best OS in the world on it (Yes, the best! Because Maemo is Debian-based)... Don't you think it's a bit funny to get rid of it and install Symbian?

This was as much as I could help... I dunno any way for doing that, but I'm sure it's possible, as N900 can do anything!