There is a great answer (with references) to this at http://answers.google.com/answers/threadview/id/539329.html which I'll summarize as follows:
From http://www.hawaii.edu/suremath/jsand.html the estimate for the grains of sand is 7.5 x 10^18 or 7.5 billion billion.
From http://www.faqs.org/faqs/astronomy/faq/part8/section-3.html the estimate for the number of stars in our own galaxy is between 2 x 10^11 and 6 x 10^11 stars.
From http://www.faqs.org/faqs/astronomy/faq/part8/section-4.html the lower end estimate for the number of galaxies in the visible universe is 8 x 10^10 galaxies.
Sand grains: ~7.5e18
Stars (low estimate): 2e11 * 8e10 = 16e21
That gives ~2000 stars per grain of sand for the low estimate of the number of stars.
Although, isn't the Milky Way on the medium to big size as far as galaxies go? If so then multiplying the number of stars in our galaxy by the number of galaxies isn't correct. However, the number used, 2e11, was on the low end so and even using something like 1e10 as an estimate of the average number of stars per galaxy still gives ~100 stars per grain of sand.
"Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space." -- Douglas Adams