bulelet,

I think you might be a little confused about what Qt is - perhaps not, but just in case, quick explanation:

Qt is a development library, that application developers can use to develop their own individual applications faster, better, and cooler.

Some applications on n900 use Qt, some don't.

You probably don't want to install Qt individually on your n900 (unless you're doing some odd kind of development with it on your device), but instead, whenever you install an application that uses it, it will be silently and automatically pulled in for you (and kept up to date afterwards, like everything else).

Hope this helps.