Better late than never...
I am also happily playing around with APEX 4.0 and busy creating some PlugIns. The first one is rather simple (once you get the hang of it). It can show you a Google Wave by just entering the Wave ID (ok, and a width and height for your convenience).
If you have a Google Wave account you can see the results and download the PlugIn on : http://tryapexnow.com/apex/f?p=PLUGINS:GOOGLEWAVE.
More to come ... : as you can see in the Wave, there are lots of good ideas on PlugIns!!