In the latest version of Safari for iOS, pressing and holding the add tab [+] button will show a list of recently closed tabs. Touching one of the listed pages will open it in a new tab.
[crarko adds: I tested this, and it works as described, although it seems a bit inconsistent in how it handles the list. Closing Safari with no open tabs clears the list, but if there are tabs still open the list is maintained. Actually quitting Safari (clicking the 'X' in the multitasking bar) clears the list whether there are tabs still open or not.
Note: As mentioned in several of the comments, this only works on an iPad.]
Mac OS X Hints