It looks like a list comprehension, just without the brackets. Before I knew about this feature, the code I was reading looked pretty mysterious.
There are some nice examples of cases where you might want to use this sort of thing in the relevant PEP. Especially pleasant uses from the PEP include passing a generator to the dictionary constructor, like so:
d = dict( (k, func(k)) for k in keylist)
... and, useful for me personally, getting the set of words in a file, all in one go:
s = set(word for line in f for word in line.split())
Good to know!