If you look at the different threads dealing with sunnies you'll notice that Serengeties are recommended more often than not. Serengeties are available in prescription lenses even for their more curved frames (such as the Lassen frame).
http://www.serengeti-eyewear.com/support/rx_program.cfm
It did take some hunting on my part to find that authentic Serengeti lenses could actually be fitted to the more curved frames; one of the main dealers around here insisted that they cannot supply authentic lenses to, for example, the Lassen frame. I think you'll find that you have to locate a dealer that gets their lenses supplied from the States as the more curved lenses do not seem to be manufactured in Europe. Another alternative is to order them online.
http://www.athleticoptics.com/defaul...orts&MfgrID=60
On a personal note, based on all the discussion here, I'll definitely be ordering authentic prescription Serengeties for my next pair of sunnies.
Cosmo