You could also use KeyRemap4MacBook, but it would make pressing return open items even in rename mode. And there are a lot of other places besides Finder where return renames items and other key combinations open them.

If you don't use a QWERTY keyboard layout, change KeyCode::O to KeyCode::CURSOR_DOWN:
<item>
<name>Use Return as Open</name>
<identifier>private.app_finder_return2commandO</identifier>
<only>FINDER</only>
<autogen>--KeyToKey-- KeyCode::RETURN, ModifierFlag::FN, KeyCode::RETURN</autogen>
<autogen>--KeyToKey-- KeyCode::RETURN, KeyCode::CURSOR_DOWN, ModifierFlag::COMMAND_L</autogen>
</item>