You can work out just expect to get tired a little quicker and not to recover quite as well. This is due to the reducuction of red blood cells caused from giving blood. Don't overdo it. It normally takes about a week or so for me to feel 100% again after giving blood. Although it can take longer for your body to actually replace the lost blood.
i worked out the day after giving blood and everything was cool. I only gave two vials though. I'm not sure about working out if you gave say, 5+ vials.