Working on an Ubercart site recently, I ran into a small snag : some of the collapsible fieldsets weren't opening under IE7 when using the Google Toolbar w/ AutoFill enabled. Specifically, the Delivery & Billing information panes weren't displaying their form fields when expanded.
The simple work-around was to go into Ubercart's checkout settings and disable the "Use collapsing checkout panes with next buttons during checkout." - this seems to be the way that all the other Ubercart sites I looked at handled the problem. A bit of investigation showed that the problem went away after rewriting theme_address_pane() to not use tables.
I'm sure there's a very good explanation for this. If it wasn't 7pm on a Friday night, I might come up with an answer.