The Swing State project has a fun, detailed rundown up of the Democrats' 2008 Senate prospects. It's worth looking at to get a sense of how things stand downticket. (I know, I know: There are elections besides Hillary versus Obama?)
Upshot: Say '08 is only a medium year for Democrats and they win only five of the ten most competitive Senate races (in '06, by contrast, they won* every single competitive race but one). And say Joe Lieberman becomes a Republican. Senate Democrats would still have widened their margin to 54 votes -- a majority that will likely be bolstered with a much, much more Democratic House, where fundraising and prospects are stronger than in the Senate. It's interesting to think about how a McCain White House would handle such a Congress.
*originally said "lost." Oops.