Whilst browsing what is on offer at wordpress.org/extend I noticed that the plugins by automattic had fancy banners (e.g. jetpack & buddypress)... I wanted in!
Since the text on these pages is generated from the readme.txt in a given plugin's repo I figured I'd take a look there and see if the automattic guys were doing anything different... oh yeah, there were!
These repo's had a "assets" folder in the root, and in there was a banner-772x250.png. Simply by generating my own banner (772px wide by 250px high), creating an assets folder in each of my repos and committing did the trick - so secret sauce required! (NOTE:You have to wait a while for wp.org to update, I waited overnight)
I think these are looking rather groovy :)