The idea of a interchangable mediums is a reality in some medium format film cameras - some models are available with the option of a "digital back" or the film back - here is an example of one model
http://www.amazon.com/exec/obidos/tg...67853?v=glance how good they are i have no idea.
Myself and some friends still use film on occasion, I have a canon 350D, nikon cp 5700, nikon cp 4500 - but the only cameras we have quality prime/fixed lenses for so far are our old pentax 35mm film cameras (we find zoom lenses not great for astrophotography) and to match the quiver of lenses in quality for the 350D dslr would take a few thousand dollars to say the least - will get there eventually tho
also when conditions are tough (ie heavy dew, dusty, frigid whatever) I am not rich or brave enough to risk my $1500+ 350d just yet (i use it for normal photography too) although i am sure it is pretty tough - so will still use the old film cameras for some of our wide field astro work for a little while
EDIT: note:we are into widefield astrophotography primarily - our guiding mount and set up is not up to prime focus work imo - plus i wouldnt want to do that manually anyway - i wouldnt bother doing 35mm film @ prime focus either - not worth it now