Thanks for the heads up. I just picked up Dishonored for $22.50 USD using that coupon [Steam download key]. The GMG site is crawling right now though... was afraid for a moment that my order wouldn't even go through. Maybe it means that they're getting plenty of sales.
GMG posted on Facebook that they've been experiencing double the traffic their site has ever dealt with, so yeah, it sounds like they're sufficiently inundated.