When I was in high school, I worked at a Sam Goody record store and learned about something called "suggestive sales." At the register, we were instructed to suggest to people that they should purchase blank Maxell cassettes (this was the late-1980s). The idea was that the suggestion of the purchase actually convinced people that they needed that item. It worked. People bought those goddamn tapes just because we'd tell them they were on sale or whatever.
That's what Senator Clinton is going with this "dream ticket" thing. She knows she's lost this thing, but she's suggesting to Democrats that they need this ticket and that she'd be at the top of that ticket. This keeps her in the running on a psychological level. She's using George Costanza's jingle: "Co...STAN-ZAHH!" The more she says it, the more it will stick and begin to make sense with voters.
The problem is... I seriously doubt she would ever allow a ticket with Senator Obama because he would entirely overshadow her and make people wish the names were reversed. So this isn't an off-hand spitball from Senator Clinton. This is a calculated psyop that should not be taken as a serious idea.