For me exporting displays doesn't work at all. I can connect to the N900 with ssh -X but $DISPLAY will stay :0. And setting it to some other value has no effect (I'd have to check again if resetting it works at all).
If I start an application (e.g. leafpad, which I think is not "hildonized" - read: broken for X forwarding) it will still not be displayed on the computer screen but on the N900.

Please try some other application that really relies on X display (xterm doesn't), something with an actual GUI! Good starting points should be any applications that are plain official Debian packages (respectively merely optified versions of these).