I just got a MacBook Pro with OS X 10.5.6, and I am having similar problems with horizontal lines disappearing on a Dell printer (model 3110cn in my case). The disappearing lines occur in a variety of places, the bar in a vertical fraction, the overbar on square roots, or even a long horizontal rule at the top of the page.
I am actually generating my text using LaTeX, which can optionally produce an intermediate PostScript file. The fact that there is an intermediate PostScript file does not change the fact that the lines are visible in the PDF file, as displayed in Preview, but they are often missing in the printout. Even more mysterious, a certain document has missing lines on page 2 of a 12 page document. If I print only the first two pages, the lines are present in the printout, but if I print the whole document, the ones on page 2 are gone. As with your case, printing from Adobe Reader works fine. Also, because I have an intermediate PostScript representation of the data, I can send that directly to the printer, and it prints fine.
I am assuming that Adobe Reader includes a fair amount of code that duplicates what Apple's internal code does, and that is why Adobe Reader prints all right.
