I'm still in evaluation/trial phase, having the 'delete project from the recent project list' problem too.
(PHPStorm 6.0 build 127.100, on windows 7)
I have no red 'x' by the selected project to click (maybe version related)
nor works the 'select project then hit delete key' solution for me
Since the second solution above was to hit a additional key simultaneously with the 'delete' key (on a mac),
i nearly tried every possible combination of additional keys (alt, ctrl, shift) with the delete-, backspace- and escape-key.
But no luck so far.
Thanks in advance