I'm not predicting this, but there is a non-trivial possibility that the Dems might just get control of all three branches. Doing this would be on the order of pulling an inside straight, but it could happen. The presidential race has clearly tightened and at this point a Kerry victory is a 50-50 proposition.
Also, the Dems are looking decent in the Senate races. Currently, there are 51 Republicans, 48 Democrats, and 1 Independent (Jeffords) who votes with the Democrats. At this point, the Democrats are likely to pick up as seat in lllinois and lose one in Georgia. The Democrats are defending 5 close seats: Florida, Louisiana, North Carolina, South Carolina, and South Dakota. The Republicans are defending 3 seats in Alaska, Colorado, Oklahoma. Of these 8 seats, the Democrats need to win 6 to take control since 50 seats, along with a Kerry victory would allow Edwards to cast the deciding vote. Right now they are leading (albeit narrowly) in 5: Alaska, Colorado, North Carolina, Oklahoma, and South Dakota. In the other 3, they are narrowly trailing. Again, it's not a given they will win enough seats to get back the Senate, but it could happen. Also, if Kerry wins the presidential race, it may help tip a few Senate races to the Democrats.
Finally, the House. Predicting House races is a mug's game given how few competitive races there are and the fact that they often turn on local and not national issues. Still, Rasmussen's tracking poll of the generic House vote currently has the Democrats opening up a respectable 6 point lead. To get a majority, they need to pick up 11 seats. That's tough but potentially doable if they catch some lucky breaks.
I should also add that these aren't independent probabilities. If Kerry wins the White House, it increases the chances that the Dems win the Senate. If Kerry wins the White House and the Dems get back the Senate, then that increases the odds that the Dems pick up the House.
Overall, I'd say that it's about a 10 or 20-1 shot that the Dems get control of all 3 branches, but I've won bets on longer odds.
If Bush wins however, it's pretty likely that the Republicans keep control of all three branches, though I could construct a plausible scenario where Bush wins and the Democrats still take back the Senate.