Thanks for pointing this out. You should only see the banner on IE6. We'll fix that.
Stupid move. If it's irrelevant to the business they should take it down. Only has the potential to turn away customers with no upside.
A more esoteric upside would be that a company that speaks out visibly against IE gives me, a technical minded customer, this warm tingly feeling inside. It suggests that someone in the company is like-minded with me. Which in turn suggests that their product and direction might appeal to me, too.
It'd obviously be a different story if this was a product aiming for joe sixpack and the mass-market. But since messaging APIs are a hard sell in the joe-sixpack market in first place I can only applaud their move; focus your resources on the product, not on the latest IE bug.