And I liked employees.select {|employee| employee.part_time?}
Crikey, should buy a book...
Personally, I prefer your version; the &: version starts reeking of Perl line noise. Although I believe Ruby would benefit from an _ for not explicitly named block arguments.
As a random note for anyone who's wondering how this syntax came about, this is the Symbol#to_proc [1] method, which was actually a monkeypatch from Rails that ended up making its way back into Ruby proper.
That was fixed some time ago. It is very marginally slower to use Symbol#to_proc now (a constant time difference to actually create the Proc), but not enough to warrant a change in practical use.