I started playing around with potential future depth charts for the QB spot.

(Eligibility remaining in parentheses. Also calling potential recruits by QB and their recruiting year. Let's assume QB10 (the 2010 recruits) never develops into starter material.)

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Scenario 1: Clausen stays for a fourth year (i.e. doesn't go pro after this year), Crist stays for a 5th.

1st String / 2nd / 3rd
2010 Clausen (0) / Crist (3) / Nate Montana (3)
2011 Crist (2) / Nate Montana / QB10
2012 Crist (1) / Nate Montana or QB10 / QB11 (4)
2013 Crist (0) / QB10 or QB11 (3)
2014 QB11 (2)

QB11 takes over with two years under his belt.

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Scenario 2: Clausen stays 4, Crist only stays 4.

2010 Clausen (0) / Crist (3) / Nate Montana (3)
2011 Crist (2) / Nate Montana / QB10
2012 Crist (1) / Nate Montana or QB10 / QB11 (4)
2013 QB10 or QB11 (3)
2014 QB10 or QB11 (2)

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Scenario 3: Clausen leaves early, Crist stays for a 5th.

2010 Crist (3) / Nate Montana (3) / QB10
2011 Crist (2) / Nate Montana / QB10
2012 Crist (1) / Nate Montana or QB10 / QB11 (4)
2013 Crist (0) / QB10 or QB11 (3)
2014 QB11 (2)

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Scenario 4: Clausen leaves early, Crist only stays 4.

2010 Crist (3) / Nate Montana (3) / QB10
2011 Crist (2) / Nate Montana / QB10
2012 Crist (1) / Nate Montana or QB10 / QB11 (4)
2013 QB10 or QB11 (3)
2014 QB10 or QB11 (2)

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Scenario 1 looks like the most probable, with Clausen staying for his senior year and Crist staying for a 5th. In that scenario, the risk exposure is in 2011 with either Montana or a sophomore QB10 backing up Crist.

The worst case scenario is probably Scenario 4.