yes that's really hard to figure out! What I always do is to use Firebug in FF if I have such issues. The nice thing is that it immediately shows in it's icon that JS errors have occurred while rendering that page. That helps to narrow it down.
BTW, did you say this worked in 4.1 but not in 4.1.1.00.23? Because I would assume this is a browser HTML parsing error and not something where APEX is involved and where we could have changed something in the patch set.
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