So, I've already posted about my issues using DDL triggers with Replication (http://thebakingdba.blogspot.com/2009/12/replication-and-ddl-triggers-do-not-mix.html). Well, it looks like it may have contributed to issues with an active/active cluster upgrade (2005 SP3).
Installing the patch on the first active node, we got a failure on the upgrade. The error message? "Target string size is too small to represent the XML instance". But, oddly enough, the passive (which is upgraded first) worked and was on SP3.
So we deleted the trigger (which, interestingly enough, showed that replication uses DDL triggers - which makes sense but I hadn't thought about), and ran it again.
It failed again, but this time because the passive node wasn't upgraded. Check both - and yes, SP3. Roll back and forth, everything looks good. A sucky upgrade, but we got through it.