Could there be an Okami sequel? If 24/7 convenience store 7-11 is to be believed, then yes, there will be one indeed. This piece of information was indicated in their online database, under the name of "Okamiden."
Okamiden, for those who have not come across this name in the past, is the same that appeared recently in a trademark database. The Kanji characters would suggest that it is indeed the successor to the original Okami title.
Don't rush to lofty conclusions just yet though. The listing from the store is not for the actual game, but is instead for the latest issue of Famitsu set to come out next week. Reportedly, the magazine will have coverage on the game. The Famitsu listing does not, however, make mention of any specific platform it will be on, if ever. It would be a safe guess, though, to say that it'll be coming to the Wii, given that Okami will be released for Nintendo's console mid-October later.
We'll keep you posted on this.