Online registration for Comic-Con International 2011 will open on Monday, November 22 at 6 AM Pacific time. This is for four-day passes that do not include access to Preview Night. Four-day passes with Preview Night have already sold out.
Last year, four-day passes without Preview Night went on sale some time in October (I think) and I just looked up to see when they sold out and found out that it was one year ago today. So I think it's safe to say that the tix which go on sale on 11/22/10 won't be around for long. At a later date, it will be possible to purchase admissions for individual days…and they'll all sell out before you know it, too.
If you are thinking of going, get your tickets while you can. If you change your mind or your life evolves such that you can't attend, you can sell your tickets back to the convention. What you probably can't do is decide next May that you want to go and then somehow get passes. You certainly can't or at least shouldn't write me a long, sad e-mail then telling me how your children will not love you if you can't take them to Comic-Con and your only hope is if I can tell you some magic way to get in. If that's the case, your kids are probably going to hate you.
You have been warned.