I don't think their relationship would be anything like or done anything like GSR...and if so, I'd probably cry :lol:. I would much prefer the subtle way of going about it. Like Drumchik said, their relationship together already is somewhat like it. Little hints here and there, to show that there is or may be something going on and leave the rest up to the imaginations of the fans.
I know there is nothing official about where they may stand with each other and again, there is a possiblity that there is nothing going on other then a healthy friendship, but we're only basing it on what we've seen thus far and of course our own personal want for it to be...well I am anyway.
In my opinion, I can't really see Reiley being in the picture for Nick and I am not about to bash that possible ship for to each their own, but personally, I think there is more chemistry between Nick and Mandy, then with Reiley. She seems more suited in personality wise with Greg...again, only my opinion.
As for that red-head in the diner at the end of season 8, well, I like to think that Nick's "supposed" interest in that waitress was just to steer possible suspision away from the REAL relationship at hand...but really do nothing about it. (oh the thoughts that go through my head)