Why would someone bother contacting you when it appears that your site is broken? In other words, the call to action doesn't display technical competence - indeed it implies a level of technical incompetence which probably is not justified.
"We're really sorry, but Paydirt isn't playing nice with your browser" doesn't inspire confidence in the product - it doesn't suggest a high level of customer service, either. Would I really want to trust something as critical as invoicing to this company?
Furthermore, not supporting IE doesn't scale well. At 10,000 users 1.6% is $1600 a month in revenue. At 100,000 it's nearly $200,000 a year in potential revenue - all for what is mostly a one time expense.
Finally, where does this leave room for expanding services such as letting my customer's see their project in real time?
I don't see a business case for it. I'm not saying that there isn't one - just that it hasn't be made.