I just ran into this same issue. It's caused by having your "My Documents" folder mapped to a different drive. The installer and program assume it's located on the C:\ drive and attempt to create or open the workspace there.
To fix the issue:
Create a new shortcut for Aptana (or modify the existing one) and add a -data parameter with the path to your workspace folder (by default under your documents). On my system it is setup like so:
"C:\Users\Lance\AppData\Local\Aptana Studio 2.0\AptanaStudio.exe" -data "E:\Documents\Aptana Studio Workspace"
Viewing the error log:
A log file is written in the /configuration/ folder of the program each time you attempt to launch the program. The log files look like they are named after some kind of timestamp (1259818956579.log). Inside the log file there is a detailed error message with the -data solution:
Root exception: java.lang.IllegalStateException: The platform metadata area could not be written: C:\Users\Lance\My Documents\Aptana Studio Workspace.metadata. By default the platform writes its content under the current working directory when the platform is launched. Use the -data parameter to specify a different content area for the platform.