I can confirm your problem - same environment - same behaviour. On WinXP and Linux. I found a bug report that matches. It was said to be fixed in M2. Seems it has come back
I too was having similar difficulties in installing JBossIDE but was able to resolve it through the following steps:
1. Clean install Eclipse 3.1.1
2. Help->SoftwareUpdates->Find and Install
3. Search for New Features to Install
4. New Remote Site
5. Add in the JBossIDE development site: http://download.jboss.org/jbosside/updates/development
and search for new features to install.
Once I did these steps, I was able to proceed without error (for now!)
The liability, of course, is this is a development version so you take your chances. You might try the ../updates/stable url instead, although I had no luck with it before.