Lookie here:
http://www.theledlight.com/ledbulbs.html They're out there, but at those prices you could have a Cateye LD1000 taillight (40-50 hours on 2AA's, steady mode) and be halfway to a 10W Niterider rechargeable headlight system too.
Very interesting web site and thanks for the link. You're right, their prices are high enought to make starting with a new light very attractive.
I have a couple of Mag Light AA flashlights but converting them to LED's would costs more than buying my 1 watt LED Princeton Tec Impact XL brand new. There are a lot of cases where upgrading older components costs more than buying a new current product.