I'm a fire mage so I probably could help out with most fire-talent-related problems. Though, if there are any other mages around who wants to help out, be my guest
I know it can be a real headache to choose which talent that you want to focus on.. may it be fire or frost or arcane, the most important thing is to know your spec well.
I'll give a brief review on various mage specs here:
ArcaneUsually, mages choose this spec for 'Presence of Mind' and 'Improved Arcane Missiles' [Channeled spells are not so good for solo PvP though, I think] ~~~ Arcane mages have higher mana durability.
FireThis spec is mainly for raids. You can still PvP in this spec tho. For fire mages, "The best defense is a good offense". Polymorph combined with a fireball/pyroblast, then continue it with a series of scorch and instant casts is mostly effective to catch an enemy unawares. This spec is also the best spec, I think, for AoE grinding/ farming (I used it to farm Lower City rep b4

) Fire mages can also solo elite mobs but so far I've only tried soloing Scholomance and Stratholme. The solo raids were a success but I've gotten bored quickly

Most fire mages need to invest some of their talent points in the arcane talent tree to gain better mana regen. ~~~ Fire mages have better sustained DPS.
FrostSo far, I have to admit that this spec is great for solo PvP vs rogue. Frost mages usually kite their enemies after slowing them down (with the chill effect). Frost mages are also good for soloing elite mobs as they can kite and deal as much damage as they could. (On BWoW, I've seen frost mages killing Level 70 elite mobs in BT b4 scripts arrived) To add it all up, frost mages also have a cool water elemental as a pet that can deal quite a lot of damage. ~~~ Frost mages have higher survivability.
I would suggest spec-ing fire to get a decent set of gear before switching to frost for maximum PvP capability. I don't exactly like being an arcane mage even though Presence of Mind can be so awesome. But still it depends on your own views.
If you guys have any problems, I'll do my best to help you out. I'm not exactly the BEST mage on the server and I've never played on retail so please pardon me if I'm wrong in any way.