Javascript
__doPostBack ('UpdatePanel1', '');
If your using UserControls the ClientId can be harder to find.
Inline .aspx Javascript
__doPostBack('<%= UpdatePanel1.ClientID =%>', '');
or
var panel = '<%= UpdatePanel1.ClientID =%>'
__doPostBack(panel, '');
A super article can be found at
http://encosia.com/2007/07/13/easily-refresh-an-updatepanel-using-javascript