You don't need to know some "hit cap" number anymore. When you mouseover the Hit rating in your character screen you now see your chance to miss against Lvl 80, 81, 82, 83 mobs.
Raid bosses being always +3 levels higher than characters are 83 mobs so you can go by this.

@nonentity: True, but when you are over the hitcap it would be nice to know how far you are over when it comes to gemming and reforging.

http://www.wowwiki.com/Spell_hit is a great source. 17% is now the spell hit cap for all casters. Right now it's about 446 hit rating. If you don't hit the cap in current gear, I'd recommend changing out all your Int/spirit gems to Int/hit now that hit is blue. If you're still not there, reforge a few pieces of armor. With some experimentation on your gear, you should be able to hit the 17.00% cap perfectly.

@nonentity, actually you still need to know the hitcap number; the new system is less helpful than the old. E.g., if it says you are 16.87%, Fine, but you still have to google to find out how much hit you need to get to 17. Same if you are over.

The problematic design is to have two different scales: the gear is denominated in points and the info box in %. Unless you show both somewhere, the customer has to alt-tab to his browser to find the conversion. Or ask the helpful folks of trade chat.