I first had this question while playing a racing game - it was a touring race with BMW 320i's, and I noticed that I could rev up to 8,500 RPM before shifting gears. Now my car (an older 320i), certainly can't hit 8,500 RPM - it can do a bit over 6,500 RPM before I need to shift (the rev limiter kicks in shortly after that).
I'd expect that one of the modifications would be adjusting/removing the rev limiter, but I also expect that if I did that to my engine as is, it would suffer. So what engine modifications are needed to enable safe higher RPMs?
Back to racing games again, in Forza Motorsport 2 there's modification which enables this - "racing cams and valves". Might this be one of them?
By the way, by "safe", I don't mean zero negative impact (I'd be surprised if an engine lasted just as long after such modifications), but perhaps minimal negative impact. Sorry for the subjectivity of this, but hopefully we can get some useful answers!
PS there really should be a "modifications" tag.