 Probability that the dems control the senate: 0.03
 Probability that there is a 50/50 split: 0.11
 Probability that the republicans hold: 0.86
UPDATE: As several alert readers have pointed out, this assumes that the results are statistically independent, which is probably not the case. However, it does suggest that our optimism may be fueled by enthusiasm as much as by history and science. The independent odds are 3%, the rest is a wave.
Take the jump for the details.
State  Current Poll Margin (+dem/rep)  Probability of Dem Victory 
AZ  7.3  .2 
MD  3.8  .65 
MI  10.4  0.9 
MO  1.6  .55 
MT  3.2  .6 
NJ  6  .75 
NV  15  .05 
OH  11.2  .9 
PA  10.2  .9 
RI  6.2  .7 
TN  7.4  .3 
VA  1.4  .55 
WA  10.8  .9 
Then I wrote a computer program that went through the probability of every potential outcome of the race. For example,
Odds of Dems winning MO MT NV OH RI TN VA WA =
LoseAZ*LoseMD*LoseMI*WinMO*...*WinWA =
.8*.35*.1*.55*...*.9 = 1.08056 *10e6

Odds of Dems winning MD MI MT NJ OH PA RI TN VA WA =
LoseAZ*WinMD*winMI*...*WinWA =
.8*.65*.9*...*.9 = 0.00758061
Then I tallied all of these up, based on whether or not the dems won more than 10 of the races (giving them control), 10 races exactly (a tie) or less than ten (republican control).
 Probability that the dems control the senate: 0.03
 Probability that there is a 50/50 split: 0.11
 Probability that the republicans hold: 0.86
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