I am having that same wilting /dying problem that I had with my tomato babes last season.
When this happened to me last year, I was sure it was the soil I used for transplanting, maybe it stayed to wet for my babies. Then someone put the bug in my ear so I was absolutely certain I had Fusarium Wilt, I proceeded to throw out all of the wilty seedlings, the leftover seeds of the wilted varieties and the soil I had used for transplanting. Then thru a veil of tears I scrubbed my entire seed room down with a stiff bleach and water solution and labeled myself a failure. about 4 weeks later I still had about 11 transplants to plant out out of my original 50 or so plants....but I was happy for those .
.....Fast forward almost 1 year to the date that this all starts happening. I know I don't have a water retention problem...I know I do let them go a little dry between watering, but the old timers and experts I have talked to say this is the best way to water.It still could be the watering , they could be getting too dry for too long(I didn't think this was the case, but one never knows) I am absolutely certain that this IS NOT fusarium wilt...so no raw bleachy hands for me ...thank you very much! The last two things I can think of are, 1- I am planting my seeds 3-4 weeks too early.It just seems like too much of a coincidence that they start acting up around the 6 week mark....if I had planted them later it would only be a week or two before they would be planted out, and I think I could save 75% of them, and for me that is a good quantity. Or 2- my house is just unsuitable for tomato seedlings. I am betting on scenario number 1 . It seems that the only issue I had with the 2007 seedlings was sun scald...and that was a faux pax on my end....I planted them mid March.
So it seems that I will be sowing my seeds a few weeks later in 2010....and if that doesn't help....I am going to find myself a good local supplier of Heirloom Tomato Transplants.....