The quick answer is yes. I have direct-seeded tomatoes in May several times with success.

Now the nitty gritty:
Your hardiness zone is rather arbitrary, because it really has more to do with the length of the growing season in your area, and the amount of heat you typically get for ripening. I live in a high-elevation zone 6 climate with a medium length growing season (120 to 150 days) and cool nights. Even transplanted tomatoes take a long time to begin ripening fruit here, and direct-seeded tomatoes might not produce anything in a cooler-than-average summer.

Your best bet for direct-seeded harvest? Try any of the smaller types, like cherry tomatoes, and anything that is said to be extra early. This year, I direct sowed a variety called "Cold Set" around the time of the average last frost (early to mid May here) and it has been producing like crazy for the past 6 weeks or so.

It's really neat that you're thinking about trying something that is a bit unconventional. However, I would say that in my experience, I'm not sure if there is an advantage to direct-seeding tomatoes. If I wanted to plant very specific varieties that were not offered as transplants, but for some reason was unable to raise my own transplants from seed (which is how I start 90% of the tomatoes I grow), then the direct-seed method would be the only way to go I guess.

In any event, I hope this was helpful.
Good luck

Excellent information... thank you!

I live in Alamogordo, NM and I think our last frost date in 2010 was during the first week of April. I will plant some of my extra cherry tomato seeds as an experiment then. Today is Oct. 21st and still no frost, so this experiment just might work!

I am planting only cherry and grape tomatoes next year. The BEST tomato I ate this year was a Bonnie Grape (sweet and tons of lovely flavor). A friend had purchased that Bonnie Grape from Lowe's Hardware Bonnie Plants collection. Her one plant was huge, healthy and productive. I would choose that, but it's not an heirloom. I only grow heirlooms.

I also obtained some FREE goat manure compost... I'm sure that will help.