Check out the new brew from Stone Brewing. It's stuffed with keffir lime and is a HUGE Belgian style. It's part of the limited series of beers that are intended to be kept 10+ years. I liked the lime...I imagine it'll smooth with age, but honestly I'd prefer coriander for a lemony zing flavor. The keffir was a bit heavy. My wife has a great keffir lime tree, though, so I may try some at some point.
I had lemongrass in a beer called Monkey Knife Fight at a pub in Philadelphia. Sorry, forgot the name, but there can't be too many lemongrass beers called Monkey Knife Fight out there
It was OK...again, I think the brewing tradition for lemony spices has focused on coriander for a reason. YMMV.