My attitude is that if the customer is paying me, then it's my job to provide them a site that supports their target user-base... and if that means supporting IE6 (common for intranets) then so be it.

I always start with developing for the current standards, and then implement additional browser support as required, either by usage stats or customer specs.

To reverse it, if I were the customer, and the developer blocked IE access with some arrogant demand they they use another browser, I'd simply refuse to pay them.