My son was born in August of 2005 and I was the same way. I was so upset about going back to work that it was ALL I could think about when I was on maternity leave. I completely changed my hair and bought a new outfit for my first day of work, so the first day held an element of excitement for me. A chance to do my hair, wear my new outfit, etc. The following day, however, I was so upset, I cried, and cried. I missed my baby so much. If you are able to go visit him on your lunch break, I found that it helped me. I also called twice a day, and still do (just to see what he was doing.)
However, what helps me is that I imagine that when he starts to talk, he will already have things to tell me. He will be able to tell me all the fun things he does during the day and all the people he sees. It keeps me so excited to see him at the end of the day. I am ALWAYS so happy to see him and hear about his day (from the babysitter, of course, not from him yet.) Anyways, I wish you the best of luck and I do assure you that it gets a little easier. I still have days that are worse than others, but then I tell myself 8 hours is not that bad! Then I will see him and enjoy my time with him.