Note that jQuery 1.3.1 did not have this problem.
I'm mostly convinced that the problem lies with the IE implementation of offsetWidth in IE. However given that this worked in 1.3.1 it might be on jQuery to have a work around. There may be other situations where IE is equally buggy with similar effects.
Note that there are some subtleties to the IE bug. For example if all the rows of the table are hidden with "display: none" AND the table is styled with "border-collapse: collapse" then the above jQuery alert with zero visible rows. However if there are some visible rows and some hidden rows I've been unable to find any css or html combination that tricks IE into the right calculation of offsetWidth.