Thoughts...
Well, essentially, the biggest pro to waiting till Lion comes out is that you won't have to pay the cost of the upgrade. The biggest con is that we aren't really sure when Lion is coming out (though we aren't sure when the updated iMac's are coming out either). The other biggest con is that with all new OS's, there are bugs in the beginning. Sometimes it's worth the wait, and the price for the kinks to be worked out before one upgrades. Snow Leopard is a well running OS at this point.
A question for those who may know the answer...in the past, when a new OS has come out, did they offer it at a discount for those who recently purchased a computer?