I downloaded the Aptana_Studio_Setup_Linux.zip package, unpacked it and run ./AptanaStudio. It starts fine, but reports one problem:
The embedded browser widget
If this is the problem I think you're having, just installing Firefox 2 (alongside FF3) should fix the issue. It happens because Aptana can only use FF2 at the moment. Hopefully they'll fix this soon.
If you're on Ubuntu, it's really just a case of:
sudo apt-get install firefox-2
Naturally, the process will vary on different distributions.