According to any release I see it's listed as June 22nd. With the Gameboy versions to release on GBA in around August. Capcom USA has the game both Gamecube and PS2 versions listed as June with the GBA Gameboy Collection listed simply as Fall.
I don't know which version of Megaman 8 is included, I hope it's the Saturn version as well. But knowing Capcom it will be the PSOne version, can let a perfectly good collection disc be to good after all. Write in and ask them about it, as far as I know they haven't told anyone (press and such). Maybe it is the Saturn version, it would be a nice suprise.
I checked Capcom Japan, they don't have it listed in either the Playstation 2 or Gamecube Line-ups. The only Megaman listings there are X7, X8, Command Mission, EXE Network Transmission, and Power Battles (both the Power Battle arcades games are being release on PS2, don't know why if they could just release Megaman Collection with them). I do know there are other Megaman Collection disc on PSOne however that are 1-8. So perhaps they have choosen not to release the title in Japan? I certainly couldn't find it listed.