Ok, I've figured out the problem and fixed it.
I upgraded the blogs Thursday night. It looks like there was a bug in the signup script. If you registered with a login name that had any upper case letters, it wouldn't let you log in. I've fixed it, so it won't affect new registrations.
For those that signed up with upper case letters, I've edited your login username to all lower case. Your posts will still show the original name with upper case letters
For example - Cassie, your login username was Cassie but now it's just cassie in all lower case. Your posts will still show up with Cassie on them
CharlottesMum, please log into your blog as charlottesmum. Your posts will also show up as CharlottesMum as you originally chose.
TBJ333, please log in as tbj333 as well
I'm so sorry for the trouble
I've updated all of the new accounts so everyone can log in. Passwords are not affected.