It took me a few minutes to get dual-head working on OpenSolaris, but once I did, I immediately found something that greatly annoyed me. Every new window I opened would launch in the middle of the two heads (half of it on one monitor, the other half on the other monitor). Also, maximizing a window made it stretch across both monitors. It turns out this bug made it just in time for the OpenSolaris 2009.06 release. To fix the issue, you have to download a patched metacity binary referenced in this bug report. Download the binary, then run the following:
pfexec cp /usr/bin/metacity /usr/bin/metacity.orig pfexec cp metacity /usr/bin/metacity /usr/bin/metacity --replace
Now, metacity should behave itself. Hopefully the devs will push out an update that resolves this soon.