Digital lenses with smaller image circles are only compatible with smaller sized digital sensors. This means that they're not usable with traditional film cameras - that much we know.
I'm interested to find out though whether anyone has (or know anyone who has) done a backflip after going digital?
I did the backflip - now I use both film and digital - for the previous 3 years I was only digital. Now I am about 75% film - I use film for ALL my "pro" work (work done for other people - not necessarily for money).
Film still has better quality and if you want to sell photos to magazines or stock houses - you need to consider pro slide films like Velvia and Provia. To get the resolution of a good slide you need about a 14 MB base image. Exposure latitude is also much better with film.
Back to the original question - because I use both film and digital, I would not buy a digital only lens - except for an extreme wide angle. On a D70 for example - a 12mm lens is equivalent to 18mm on 35mm. I don't need a 12mm lens for my film camera - so I "might" consider a digital only lens for the super wide angle niche. My widest current film lens is 24mm - a sweet angle for film - but totally unacceptable as a wide-angle solution for digital (36mm).
I'm actually seriously considering buying my first film SLR after getting much more interested in photograohy since I got the rebel. I want to enter slide competitions and film is the only sensible way to get slides made.
Digital to slide conversion is way to expensive.