It doesn't appear to have been mentioned in the well-informed blogs and opinions I frequent, but one downside to a Kerry victory is that he would have to give up his Senate seat. Mitt Romney, the governor of Massachusetts, is a Republican and nothing would lead one to suspect he would feel obliged to appoint a Democrat to maintain the partisan balance in the Senate.
On balance, of course, it would be far better for Kerry to be President. Moreover, Kerry should pick his VP with an eye to winning the Presidency rather than holding the Democratic position in the Senate.
In the "worst case" scenario where Kerry picks a Senator from a Republican governed seat as his VP and they win, the Democrats will have to pick up two additional Senate seats to regain control of the Senate.
At present the Democrats are down 48 (+1) to 51 in the Senate.
To gain a working majority, they need to pick up 3 seats, and given that they're likely to lose Georgia and South Carolina, this requires that they pick up another 2 to compensate for this loss. Assuming that Kerry and his Senator-from-a-Republican-governed-state win, then the Dems have to pick up another 2 seats.
On most readings of the races, just 4 Republican seats are vulnerable: Illinois, Colorado, Alaska and Oklahoma. I'm not exactly high on the Dem's prospects of taking out PA, MO and KY which would compensate for the 2 Senators in the White House and losses in the South.
Presidential coat tails are not likely to be very long this year, on either side. For better or worse, you really have to do something egregiously wrong or incompetent to be dislodged from a Senate seat once you're there.
In summary, it's hard to see the Democrats picking up enough seats to regain the Senate, given that if Kerry wins they're down at least another one.