As ever, at the end of the season, your conquest points will be converted to honor points. In the interim period between Aug. 28 (29 for EU) and Mists' arrival on Sept. 25, there will be no honor cap. Come Sept. 25, all your points over the 4,000 honor point cap will be converted to a ridiculously tiny amount of gold -- 35
On Aug. 28 (29 for EU), all the gear that was purchasable with conquest points becomes purchasable with honor points. Any items with rating requirements attached are no longer available for purchase.
If you think you may be in the running for an achievement, it is very important not to do anything to the character you think might be eligible. Don't faction-change, don't realm-change; either of these things could wipe out your rating.
Season 12 will begin approximately one week after the release of Mists of Pandaria, and in the interim period, there will be no arenas or rated battlegrounds available. Only normal battlegrounds will continue as usual.