Blizzard poster Bashiok has just posted an in-depth explanation of the PvE/PvP currency system changes that are coming in Cataclysm, and it's an informative read. The lengthy post can be found in its entirety after the jump, but I'll attempt to summarize the important parts.
The upcoming 4.0 patch will convert Emblems of Triumph and Frost into Justice Points (PvE), and Arena Points, Honor Points, Battleground Marks of Honor, Stone Keeper's Shards, Venture Coins, and Spirit Shards into Honor Points (PvP). Simple, right? All PvE badges below Frost and Triumph will be converted into gold.
There will be a limit to the amount of new currencies you can hold, but Blizzard's providing a grace period in the form of a soft cap. This means your conversion can go over the hard cap, but you can't earn more until you go below the hard cap. Here's an example: the conversion gives you 4,200 Justice Points, but the hard cap is 4,000. You'll have to spend some points to dip under 4,000 before you can earn more. Another patch before Cataclysm will actually enforce the hard cap, and your excess points will be turned into gold.
Keep reading below for more details. Bashiok said that all conversion math should be posted tomorrow afternoon if you're curious how many Justice and Honor Points you're going to earn in patch 4.0.