Well, you can open the preferences (Window > Preferences) and then go to General > Editors > File Associations. Click the top "Add" to add '*.erb' and then, with it highlighted, click the bottom "Add" to associate the rhtml editor with that extension.
But here's the downside... I just tried it and the .erb file opens with the rhtml editor, but it looks like it's still being treated as plain text.
Well, I have a fairly old version of the eclipse radrails plugin... should download the latest from Aptana and try it.