Pool users shouldn't have to update anything. Pools build the block they hand out, users just try to find a nonce for it. You should know this if you run a pool. Additionally, there is a development mailing list you can join where this was heavily discussed:
http://sourceforge.net/mailarchive/forum.php?forum_name=bitcoin-development
http://sourceforge.net/mailarchive/forum.php?forum_name=bitcoin-development
notme is exactly right; the change is backwards-compatible, pool users don't have to do anything.
Pools and solo miners should upgrade, or they run a (very small) risk that they'll waste time hashing a block that can't be valid.
The risk is very small because it requires that somebody mine a block containing a /P2SH/ transaction that is valid-under-the-old-rules, invalid-under-the-new. That won't happen by accident, somebody malicious will have to create such a transaction and then find a miner who is willing to put that non-standard transaction in their block (and is willing to create a block they know the network will reject).
They would spend a lot of time (and therefore money) on an attack that would do nothing but slow down transaction confirmations a tiny bit and maybe trip up some random, unlucky mining pool or solo miner who didn't bother upgrading.
Gory details if you're not already bored:
Old miners and clients will ignore all /P2SH/ transactions; they won't relay them to other nodes and won't put them in blocks they mine, because they're non-standard. So an attacker can't broadcast an invalid /P2SH/ transaction and hope it gets included in a block; they'll have to mine a block themself, or partner with a big solo miner or pool who is willing to produce bad blocks.
If an attacker DID manage to create a block with a timestamp after the switchover date and a bad /P2SH/ transaction in it, then some percentage of the network will try to build on that bad block. Lets say 70% of hashing power supports /P2SH/. That would mean only 70% of the network was working on a good block-chain, and the result would be transactions taking, on average, about 14 minutes to confirm instead of the usual 10 minutes.
In other words: they'd give up a $300 block reward and manage to just give the network a tiny little hiccup.