I started with the stormcloaks. Killed the ice wraith, and now the jagged crown quest is atarted in my quest log. I haven't advanced any further than that. Can I switch sides?

I think you cement your membership based on who you give that crown to.

edit: Or maybe who you help to retrieve it....

According to UESP you can choose who you give the crown to so this is the last point at which you have a choice of sides

see if you still have "join the imperial army" and "join the stormcloak rebellion" in your misc quests, if they are still there you can, otherwise its too late

Yep. You can go up to Solitude and hand the crown over to Tullius.

Thanks guys. Just gotta wrap up a few lingering quests and i'll continue the civil war.

I didn't know this, interesting