Gems
Socket bonuses appear to be mostly based on Intellect, and it seems that most of socket bonuses will be worth gemming for at first glance. The desirable gem(s) for each slot are as follows:
Red: Brilliant Inferno Ruby
Blue: Purified Demon's Eye
Yellow: Reckless Ember Topaz
Meta: Burning Shadowspirit Diamond*
Only rare gems exist as a part of Cataclysm. I expect epic gems to make their way back into the game in a future patch. Also, if a unique +20 all stats gem, like Nightmare Tear, makes its way into the game again, it would be ideal for a blue socket. I would highly recommend not gemming for Hit. You will gain more by gemming for Intellect and reforging secondary stats into Hit/Spirit if you need the rating.
You should gem for the socket bonuses if the bonuses are:
Red Sockets = Always 40 Intellect in a red socket
Yellow Sockets (20 Int + 20 Haste) = Intellect > 9. Haste > 19. Crit > 23. Mastery > 22
Blue Sockets (20 Int + 20 Spirit)* = Intellect > 19. Haste > 40. Crit > 45. Mastery > 45
*This assumes you can make up the hit through reforging and avoiding a loss of Intellect by gemming for Spirit