When I look in the console I get this message
2010-09-18 17:04:05.284 Wasted Time[8998:207] *** Assertion failure in -[UIActionSheet showInView:], /SourceCache/UI
I also had this problem but I was using [actionSheet showInView:[self.view window]]; (to fix another bug where only half the cancel button is selectable).
Now I conditionally change this to [actionSheet showInView:self.view]; on the iPad.
I found it's best not to the style to UIActionSheetStyleBlackTranslucent on the iPad either – the iPad has it's own style, and setting this seems to add the cancel button (which by default on the iPad is not shown).