Xrystal is correct... there's a slash command you can use to turn action bar tooltips on during combat (they default to off)
The complete list of slash commands is here:
http://www.wowinterface.com/forums/s...ad.php?t=21199
The one you're looking for is this...
/nui combattips
This option toggles display of action button tooltips on and off while in combat lockdown. The default is to hide the tooltips in your action bar buttons while in combat.