I think the entire shift thing simply confuses people.  I actually ignore the shift concept when I look at things.  I think if there was a way to eliminate shifts and get paid for your contribution per block, it would simplify things.

block 151 start
12:01am
1am 0 hashes
2am 0 hashes
3am 134 hashes
4am 5000 hashes
5am 576 hashes
6am 0 hashes
7am 0 hashes
8am 0 hashes
9am 0hashes
10am 0 hashes
10:33am block 151 found
10:33am block 152 starts
11am 0 hashes
12:01pm 0 hashes
ect ect ect

say the hunt for block 151 started at midnight and it was discovered at 10:33am hour.
Lets say a pool hopper jumped in during the 3am hour and left during the 5am hour.
His total contribution would be 5710 hashes for the time he spent here.
Even though his time here was short he still contributed some hash to the pool.
His share of pay would be equal to the percentage of his contribution.
Each block requires a different amount of hash to discover.
Say this block only took 11,000,000,000 hash to find.
His pay would be 5710/11,000,000,000*25btc=0.0000129772727273

Now say that block took 24 hours to find, and it took 37,000,000,000 hashes to discover.
The hoppers cut would go down because he only contributed 5710 hashes to that block.
His pay would now only be 5710/37,000,000,000*25btc= 0.0000038581081081

Since the blocks being found is simple chance of luck, time is irrelevant other then to see
How long its taking to find the next block, thusly making shifts pointless in my opinion.