When started from within Emacs (e.g. "M-| nopaste RET" on a selected region) the process never returns. "ps x" shows that there is an xclip process left that doesn't terminate (because its standard output is going to a pipe, too). Patch will follow.
Created attachment 140822 [details, diff] Proposed patch for nopaste-2802 Redirecting stdout fixes the problem. (Maybe this is also needed for xcut, but I didn't bother to investigate since xcut was removed from the Portage tree in bug 191553.)
Also sent to upstream (i.e. agriffis@n01se.net).
Fixed (and version bump to 2835) in CVS.