you cant install the tarballs provided by nokia at qt.nokia.com on the n900, because those are not compiled for armel
what you could do: download the source, compile it by yourself with the n900 dependency libraries, and replace the default qt instalation with that one.
you will break the existing applications, because those are not abi compatible
but if you know what are you doing, there are workarounds for that
anythoughts?