There already was, sorta... A televised miniseries which you can obtain on video or DVD as a single feature, it was... not very good in terms of production values, but a quality time none the less.
The story evolved so many times that there is no single "right" way to do it though... The original radio series was different than the second run of the radio series, was different than the TV series, was different than the first release of the book was different than subsequent releases of the book was different than the last radio series...
What worries me though is that Douglas Adams passed away a few years back and, while all of the previous variants were different in subtle ways, they were all HIS creation and any changes were made according to HIS decisions. While is is possible for a director to create a motion picture that's true to the original author's ideas (i.e., Fear and loathing in Las Vegas) and any changes are subtle, it's not common.
As a fanboy I'll probably be dissapointed... but I'd definately see it regardless.
"Genes, Like Leibnitz's monads, have no windows; the higher properties of life are emergent... And once assembled, organisms have no windows." - Edward Wilson, Sociobiology