Jeff Hopkins does exist. He worked at Universal until 2002, when he was fired (must have been in HS at the time).
There's a myspace page for him that says he's 23, and there is a Jeffrey Hopkins that graduated from Colonial in 2003, the year before Casey.
http://www.colonialhigh.ocps.net/alumni/2003.htm
According to Cindy's testimony, Casey told her that Zenaida had been babysitting for Jeff's child, and started sitting for Caylee about 18 months ago.
Jeff's myspace page shows that he wants kids "someday", which doesn't sound to me as if he'd have need of a sitter.
Cindy also testified that she thought Caylee had gone to Jacksonville with Jeff Hopkins during the time she was missing. Now that we know Cindy was able to track Casey's movement to an extent because she was using Cindy's credit cards, I'm placing more weight on her knowledge that Casey may indeed have been in Jacksonville at some point (with or without Jeff).