Of course the problem with this strategy is that you may not be in a vacuum. If I'm a competing shop I simply say that I'll do it all for a price somewhat south of your +IE6 price.
And if I have some experienced webdevs who have done this type of stuff since forever, the cost is already largely locked in. Sure I may not be "helping the web" (whatever that means), it may mean a substantial boost in revenue.
I recently had out a RFP for some construction work. One guy called me and told me to call him back when I've got some offers, he'd knock 10% off the lowest one.
Needless to say, I said thanks, hung up, laughed and crossed his name off the list of candidates.
Then you've dropped too low. You certainly don't do it for $1. But I've found that whenever there are other companies with ideological bents (like anti-IE6) I can almost certainly always crush them economically and with respect to quality.
Competing aggressively on price, more often than not, attracts a certain type of client. If you're taking the cheap option now, you'll often fight over every cent going forward and be pretty miserable to work with.
If someone is desperate for work, they might need to make sacrifices to keep busy. I'm not, so I try not to.
It's not all that aggressive. If your intention is to effectively price out technology X (IE6), I can almost always beat your price by a good mile, and I'll make a point of it.
I think you underestimate the huge price differentials for almost no difference in quality. In fact often quote the opposite. I've seen a factor of 5x in price difference in some cases with virtually no difference in portfolio, mockups, or code quality.
That's fine, but I (a hypothetical web designer) am selecting for clients who don't actually care to do IE6 compatibility work, but would try to get it if it were the same price.
As another poster mentioned, it's a demographics issue. Right now, I'm working on a site used in US K-12 schools, and IE6 usage is around 5%. It doesn't sound like much, but our client can easily justify supporting IE6 based on marginal revenue. We have a large Google Web Toolkit application, so IE6 wrangling is limited to CSS and performance hell -- the JS differences are compiled away.
If you're developing a consumer app, you're probably in pretty good shape though. Some friends at a consumer-focused webshop report IE6 rates < 3%. Furthermore, most of that 3% probably have Firefox installed and could use it easily if they were told that the site wasn't IE6 compatible. So, for them, the ROI of IE6 support is probably quite negative.
And if I have some experienced webdevs who have done this type of stuff since forever, the cost is already largely locked in. Sure I may not be "helping the web" (whatever that means), it may mean a substantial boost in revenue.