One thing, at least at this point, that's really interesting about the 2010 Senate picture is the sheer number of races that look like they will be competitive.
In each of the last three election cycles there were seven Senate races decided by less than ten points. But at this very early stage I would expect at least 14 seats could reach that level of competitiveness and seven of them are held by each side.
Republican held: Missouri, Kentucky, New Hampshire, Florida, Ohio, North Carolina, Louisiana
Democratic held: Illinois, Delaware, Arkansas, Colorado, Connecticut, Nevada, Pennsylvania
New York and North Dakota could be added to the Democratic list as well depending on candidate recruitment. I'm still skeptical about California but we'll see.
Because there are so many somewhat competitive seats on each side of the ledger there really is an extremely wide range of possibilities for what could happen next year. Certainly in the last few election cycles most of the close races have all ended up going in one direction or another so you could see Republicans whittle the number of Democratic seats down to the low 50s if they have a good year as expected but if somehow things get turned around for Democrats to have a good year they could climb up well into the mid 60s.