You may use the T flag to use the form HH:MM[:SS]. Example:
| Start | End | Days | HH:MM:SS |
|------------------------+------------------------+----------+----------|
| <2013-07-15 Tue 10:15> | <2013-07-17 Wed 11:15> | 2.041667 | 49:00:00 |
| | | 0 | 00:00:00 |
#+TBLFM: $3=date(<$2>)-date(<$1>)::$4=60*60*24*$3;T