I just thought I'd pass on Stephanie's response, as she couldn't post to the list:

It looks like the SIGPLAN class file has gotten out of sync with the paper requirements and is producing a slightly too large textblock. I just checked the template (filled out with random text) against the format checker. It fails.

In this situation, it seems wisest to go with the class file instead of the stated height requirement. I've upped the format checker to a max 9.5in height to account for this discrepancy.

Hope this helps, Wouter