Last year, I started seedlings on September 18 and had them outside by October 25. If you remember, we had a brutal hot spell right around then, and my non-resistant heirloom plants were infected by septoria fungus right away. I ended up losing a few plants early in the season and spending the rest of the season battling this stuff with increasingly strong chemicals. So if anything, I'm in favor of a slightly later start to avoid the worst of the hot and wet fall weather. That said, I don't seem constitutionally capable of waiting much past the third week of September.
If, however, you're buying tomato plants, I'd still recommend at least another two weeks, if you don't have plants already. If you do have plants already, no big deal—just hope it cools off before they start to set fruit. Many tomatoes won't set fruit too far above 80˚F.
Anyway ... to the seedlings! This year, I'm trying something a bit different with my seedlings. In the past, I've always grown seedlings the same way: in peat pellets or 2" pots with a simple and basic mix of about 70% composted peat moss, 20% perlite and 10% worm castings, under compact fluorescent grow lights. It's worked like a charm, so there's no reason it shouldn't work for everybody (just make sure your lights aren't too old—they lose lumens pretty fast).
This year, however, I have a new(ish) toy I'm taking for a test run. A few years back, I got an AeroGarden as a test unit and I've used it to grow indoor herbs during the off-season. It's a nifty little unit, and you've probably seen these things in stores. They are basic, desktop hydroponic grow units. It's not advanced hydro and it's not complicated. (If anything, the downside is expense—I think the units cost a few hundred bucks each.)
AeroGrow also makes a seedlings starter kit. The concept is pretty simple: it's a styrofoam grow plate that floats on a shallow pool of nutrient-enriched water. A fish-tank air stone is used to aerate the water. The unit includes lights. It allows you to grow up to 66 seedlings in a small space. So what the heck, I figured, I'll give it a try this year. I'll keep you posted with pictures and progress as the plants sprout (early notice: the mustard greens literally sprouted overnight ... how satisfying is that?).
In general, this year I'm hoping to try a few new things, and I think I'll have to really revisit the question of organic versus non-organic growing methods. My own thinking on this topic is far from settled, and I've received a few letters from people who thought I really stepped on the kitten last year by switching to chlorothalonil in mid-season to control the leaf-spotting. I had another very, very good grower—someone I respect a great deal who makes a living from this—recommend preventive spraying with neem oil, an organic insecticide, bactericide and fungicide. I dunno yet. Right now, I see this as the central challenge of the season.
Mostly, though, I'm pretty excited. I've already developed the bad habit of checking on the seedlings like twenty times a day ...