This makes a pretty good argument for having a large number of Bitcoin processors ready to accept physical cash in exchange for Bitcoins.  I could see a large-sized city supporting a Bitcoin processor.  Heck, it doesn't have to be a full-time job; just have specific posted hours when you're exchanging Bitcoins for cash.  It'd still be a lot faster to drive somewhere and exchange cash than wait for something like Dwolla to work out.