The autocompletion selection key(s) is editor specific. For instance in Aptana Studio, you could define set of keys for proposal selection. See e.g. Preferences/Aptana Studio/Editors/JavaScript. PyDev shares some code with Aptana Studio, so it has similar capabilities. See "Preferences/PyDev/Editor/Code Completion/Apply completion on *" checkboxes.
Speaking of the Tab key as a selector, it doesn't seem possible w/o extra coding since it has special meaning to focus into proposals window. For these who interested could find Tab handling at org.eclipse.jface.text.contentassist.CompletionProposalPopup.verifyKey(VerifyEvent)
Cheers,
Max