Counter service location hours may vary based on the season. At particularly busy times, locations tend to open for lunch service earlier. When you enter a park, pick up a Times Guide and it will tell you what most of the counter service location hours are. Usually each park will have one location that tends to open for lunch earlier than the others.
Going from my (not always reliable) memory:
AK: I *think* Flame Tree but I'm really fuzzy on this one. It's been a while since I've been in this park.
ST: IIRC ABC Commissary switches from Breakfast to Lunch around 10:30am. I know during Christmas I've seen Sci-Fi start seating at 10:30am. If all else fails, Starring Rolls should be able to sell you a sandwich by 10:30am.
MK: According to the 6/8 - 6/14/08 times guide I excavated from my backpack, Cosmic Ray's opened at 10am while Columbia Harbour House and Pecos Bill's both opened at 10:30am.
EP: Sunshine Seasons opens for breakfast with the park. I think it's around 10:30am that they start opening up the other stations for lunch service and start putting out the grab n' go lunch items. The Asian station is the one that serves breakfast so it will be the last station to start serving lunch items.