Skill Challenges With Organic Time Limits in 4E

So I've been thinking about skill challenges again. One tweak I've come up with is to add in an organic time limit, by way of two steps:

• Calculate how many rounds a normal skill challenge would take at maximum, if all rolls contribute either one success or one failure.
  
• Add the number of complete rounds to the number of failures needed to fail, but also add one failure per round that passes without the PC:s completing the challenge.
  

Example: A complexity 5 challenge requires 12 successes before 3 failures. Thus the maximum number of rolls is 14 (11 successes, 2 failures, the 14:th roll decides the outcome either way). A five player party would complete two rounds and be at their third when the challenge completes.
Thus, the modified skill challenge requires 12 successes before 5 failures.

If the PC:s only roll for primary skills without any tricks, this is mathematically identical to the core skill challenge rules, but it opens up several possibilities:

• You can have secondary skills that grant bonuses to primary skill checks, but rolling for them takes you closer to getting an "extra" failure for taking an extra round.
  
• Spending action points to roll for those secondary skills becomes interesting, turning skill challenges from a "free" half-step towards the next milestone into an actual encounter that drains resources.
  
• Skill rolls where success/failure count as two of whatever you rolled become more attractive.
  
• And of course, a counter-incentive to skipping your turn, since you're usually not better off not rolling now, even if your skills are bad for the situation.
  

I think I might try this for the next skill challenge I run.