I am going for something like
http://www.wowhead.com/?talent=of0E00bZZVqGfRfxobot. That is: sacrifice some frost talents to get Improved Counterspell, which comes in quite handy in Pve and is extremely handy in Pvp. The improved counterspell talent gives your basic counterspell (which only of use when target is casting something) additionally a true silencing effect (only 4 seconds, but thats quite a lot). I lost track of the number of times that has saved my behind. I am not sure yet *where* exactly to sacrifice frost talents though - the linked build is one proposal.
As for pvp: I have both a highlevel frostmage and a highlevel firemage. Though I didn't really even take my fire mage into pvp situations (I play on a pve realm, so pvp means battlegrounds), I can quite safely state that a frost mage has much more survivability in pvp. A few reasons:
1) Ice barrier. For three reasons I think this is the crown of the frost talents: Effectively it adds a significant percentage to your hitpoints, it prevents all interruptions to spellcasting from damage (not interruptions from silencing effects, of course), and as a bonus: the 'extra hitpoints' from ice bariere are the first to go down, but can't be healed/don't need to be healed - your party's healer can spend his time healing others. These things combined mean that you are much less reliant on +STA gear than fire mages, and you can get +dmg gear instead.
2) Ice block. Not too useful in solo situations (except if those 10 seconds buy you the time to use another cooldown), but great in both party pve as well as party pvp. It allows your tank to regain aggro, or the other fella's on your side to dispose of whatever was bothering you in pvp.
3) Your bread and butter spell (frost bolt) also slows down your enemy, making it much easier to keep distance, and buying you time to get more damage delivered.
One subtlety: I noticed that the cooldown reduction provided by Ice Floes is really handy in hot fights. Not so much the cooldown on ice block or cold snap, but the cooldown on Ice Barrier means you can recast it every 24 seconds (instead of 30), which has meant the difference between to slay or to be slain for me quite frequently, both in pve and pvp.
Tonster wrote:I don't think a frost mage would want to face a warlock though
That depends. A warlock with his felhunter out basically means 'game over' - you get silenced, your Ice Barrier, Frost armor, Arcane Int, and Dampen magic get not only dispelled, but each dispel healls the felhunter. With other pets you stand a chance. Especially considering you can remove curses (from yourself)