i think its a good idea, definitely should be explored. the gnome project recently (with the help of novel/ximian) setup some "bounties" which seem like a good way to do this:
http://www.gnome.org/bounties/
another method is that like the freenet project uses: they hire a fulltime developer paid entirely by donations to a central fund. as i think olaf pointed out, this requires someone with the power/will to decide who to hire and how to rate their performance... sounds somewhat political and sticky.