I can see from one standpoint that it would be hard to rememeber to put in the hypen but I think the search engine might be better on the hyphenated.
No hyphen. Perhaps for SEO (Only "perhaps"), but for every other possible commercial use, no-hyphen. And I'm not-very-hyphen-averse
http://www.petstore.com/ -> 301 REDIRECT http://www.petstore.com/pet-store.htm You get both: no hypen for easy rememberance for users, hyphen for the "just in case" SEO angle.
Good thought there... I have also seen that google picks up the words even without the spaced. Good luck.
In my opinion if you have petstore.com hold on to it, pet-store.com isnt as premium i could see petstore.com go for thousands on sedo easily.