How to format a duration as HH:mm in the new Google sheets

There is no need to use formulas for that, you can define your own custom formats.

Just go to Format -> Number -> More formats -> More date and time formats. It will open a window with several date and time formats to choose from. You can define your own as well, using the upper text area:

1. Click on the small arrow on the left, and select the first element you want to add: elapsed hours. Notice there are two different "hours", one for time and another for durations.
2. Type your separator text :
3. Click on the arrow again and add the second element: Elapsed Minutes.
4. Finally, click on each element you added, to determine the correct format (trailing zeros or not, for example).

In Excel it's "Custom" then [hh]:mm.

(I'm not quite sure how I figured this out, but it's in a spreadsheet I've been using for a year or so.)

The answer from Hot Licks and spex worked for me in Google sheets - enclosing the h in [] , as in TEXT(A2,"[h]:mm:ss"), allows a duration larger than 24 hours. Without the [], 28 hours shows up as 4, with the [h], 28 hours shows as 28.