They end up having sex during which a small part of Steve's conscience knows something is very wrong but Loki just dials up the 'I'm in mortal danger, please protect me!' rhetoric and Steve succumbs fully to Loki offering his body as a means to repay Steve for his help.
Loki then basically uses Steve as a bodyguard to escape the tower and very nearly succeeds but the other Avengers manage to thwart his plans and subdue Steve who doesn't hold back in fighting them.
I remember it ends with everyone thinking the status quo has been fully restored but it turns out Steve is actually still on Loki's side...
I'm almost sure this was a meme fill, either norsekink or avengerkink? If there's any way of easily searching the fill lists for those memes please let me know! :)
FOUND IT! Started scrolling through the fill list on norsekink and found it on page 24 http://norsekink.livejournal.com/2496.html?thread=2790336#t2790336
Now I'm off to read the prequel mentioned in the fill comments and which can be found here