How soon is TOO soon?

I can't answer on the medical side - really, you need to talk to your OBGYN about that before you take any advice from anyone online - but one thing you should think about is whether or not you want your kids to be close together in age.  I know I would much rather have my 18-month-old potty-trained (at least in part) before we had another child. 

If you're in a mommy group, ask the other moms (especially if there are some with older kids) about the pros and cons of having kids close together v far apart.  And ask your friends, too, how they felt about having siblings close in age or much older/younger.  Close in age doesn't always equal a close relationship.  My brother is 18 months younger than me; we have never really gotten along.  My husband is 4 years older than his brother; they talk three to four times a week.