The Surly Big Dummy has been a staple of long tail cargo bikes for some time now. I have my own Big Dummy I plan to feature on the site in the future that was converted to Fat Capable, although it has lots of limitations: Not offset, still 135mm rear spacing, not compatible with many of the newer fat bike wheels, cranks,etc.
Surly has been a proponent of Fat Bikes long before Fat Bikes became an industry fad, so it is only logical to combine the two for the Big Fat Dummy. Rolling out to dealers spring 2007 I am excited to see these rolling through neighbors as grocery getters, kid haulers and all around car alternatives. Get more details at Surly's site here