It looks very similar to what I get every year, some kind of fungal disease.

Here's a link to a good Penn State site for diagnosing turfgrass diseases.

Here is a link that might be useful: Turfgrass Diseases

That looks very similar to the sod web worm damage I had. Does the lawn actually die out in those brown spots?

Did you ever find out what your lawns problem was? I have very similar looking brown spots..my grass is tall fescue..and I am also in zone 5

It's hard to tell from those pics, but isn't that Red Thread? I think I see some mycelium strands in that first pic.

Either way, sure looks like a fungus to me.

Since this is the Organic forum, I'll suggest trying plain corn meal (not corn gluten). It worked for me, but it's hard to spread.

Looks to me like leaf blight. Without using a fungicide, these types of diseases are best avoided by using disease resistant grasses like Tall Fescue, and aerating frequently... all diseases occur when you have moisture and susceptible grasses

A simple easy way to tell it this is grub damage is to take a spade and cut into the turf in a "U" shape and peel back the sod and count the number of grubs you find in that approximate 1 square foot. 5 or more may indicate a problem, fewer means there is not a grub problem.
A simple easy way to determine if this may be Sod Web Worm or Chinch bugs is to take a can, about the size of a 3 pound coffee can, with both ends cut out and stick it down in the soil (it does need to be in the soil but not very far) and then fill that can with water and look at what floats.
Unless that is much more extensive than it appears in those photos it is not something I would be concerned about and certainly would not arbitrarely apply some kind of fungicide or insecticide without knowing just what is the problem. Your county office of your state universities USDA Cooperative Extension Service will be able to help identify what you have, and can tell you if it is a real problem.

can anyone tell me where i might purchase nemetoads - I am trying to control a serious grub/JB problem without resorting to chemical (though I am getting pretty close...)
Thanks

Click here for a source of nematodes.