PSX4Droid, has been pulled from the Market. The reason? Its author, ZodTTD, said Google informed him that it was a matter of "Content Policy violation," however the fact it's taken eight months for Google to take action suggests it might have a little something to do with the imminent launch of the Xperia Play, which will have its own mechanism to power the original PlayStation's games. Another mystery is that it looks like other emulators such as SNesoid (SNES), GameBoid (Game Boy) and, in particular, FPSe (PlayStation) appear to be safe for now, which might suggest that there are other reasons behind this removal; but still, the timing suggests otherwise. On the bright side, we can always rely on third-party app markets.
Update: Here's a super generic statement we just received from Google regarding this matter:
"We remove apps from Android Market that violate our policies."