The ESRB has recently given new ratings to the following PS1 titles, indicating their imminent release on the PlayStation Network:
Contra: Legacy of War (1996)
Fighting Force (1997)
Medal of Honor Underground (2000)
In addition to that, the ESRB has re-rated Rayman 2: Revolution (2001), a PS2 enhanced remake of the PS1 title Rayman 2: The Great Escape (1999). It is unknown whether or not the PS1 or PS2 version will be released. Needless to say, if the PS2 version is ported and therefore made compatible with all PS3 and PSP systems, this could be the start of other PS2 titles making their way to digital distribution.
In the case of Rayman 2: Revolution, the ESRB rating appeared back in 2008 and was thought to be a mistake of some sort. However, the fact that the rating remains under the PS2 title after over half a year makes the promise of PS2 games on PSN seem more likely.