You can:
O Freely upgrade PS3 hard drive
O Install multiple third-party OS on PS3
O Play any region's PS3 games
You can't:
X Import any PS3 games
X Play any region's PS1 and 2 games on PS3
X Run homebrew on PSP (without exploiting loopholes)
There are so many contradictions these days. They give you some freedoms, take some away. The PS3's allowing multiple non-Sony OSs is pretty remarkable, considering.