Eclipse is a solid tool for creating Drupal modules, but I haven’t figured out anyway to use it to create releases. Eclipse comes with a CVS plug in that allows you to checkout the contributions/modules, add your new module and commit it all, no problem. But when it comes time to branch your release(as described here: http://drupal.org/node/17570), simply trying to do: “cvs tag -b DRUPAL-5 modules/mymodule” seems to be impossible. Eclipse is not willing to branch with out also tagging the version you are branching from, and the Drupal contrib release system rejects these verison tags. It seems to be a clash of philosophies. The Eclipse guys believe you should never branch without tagging where you are branching from. And I have not found any way to just issue a custom cvs string. Frustrating.

So I have turned to WinCVS to do the dirty work.
Select: Remote->Create a branch…
New: DRUPAL-5 (or whatever)
Modules: contributions/modules/your_module
CVSROOT: :pserver;username=your_name;password=your_pass;hostname=cvs.drupal.org:/cvs/drupal-contrib

Update: Can’t branch from Eclipse, but you can tag release version, just use Team->Tag as Version… (ex: DRUPAL-5—1-0)