On Sept 1rst 2011 at 00:12:51 UTC (in under a week), the new client will switch over to a new Ixcoin-specific pchMessageStart peering marker from the original Bitcoin marker. This change was recommended by a few although we haven't had reports of any issues with this yet. Better safe than sorry. The updated Ixcoin nodes will not be able to communicate with non-updated Ixcoin clients after that date. This will require a restart of the Ixcoin client on or after Sept 1rst. (commit)"
That only stops the two halves of the network from communicating directly; it doesn't prevent blocks generated on one half being valid on the other if you can transfer them over in some other way. In fact I've tested this with a pair of nodes locally and 0.3.24.2 does appear to consider the longer 20032-block chain perfectly valid, I just can't get it to propagate across the network for some reason. Something's really quite broken...