It could be virus, or it could be a PH Imbalance, or it could be a nutrient shortage, iron comes to mind. If you would post some information as to how you are growing the plants, inground? containers? and what have you added to the soil we could make some educated guesses. Also is the yellow growth normal looking otherwise?

Also, if it helps with a diagnosis, the yellowing is not at the leaf tips. Its heaviest where the leaf is joined to the branch, and then spreads outward. Its a relatively pale yellowing, and uniform (no brown spots or anything). Thanks!

Cdon, have a look at these photos of iron shortage:
http://vegetablemdonline.ppath.cornell.edu/DiagnosticKeys/TomLeaf/Iron_Tom.htm
and the next-to-last photo on this page:
http://5e.plantphys.net/article.php?ch=t&id=289

Its heaviest where the leaf is joined to the branch, and then spreads outward. Its a relatively pale yellowing, and uniform

Herbicide damage is what that sounds like. No way to be sure without seeing it but it is a common problem this time of year. You can see pics of it on Goggle Images to compare with yours.

Dave

Here is a link that might be useful: Herbicide injury to tomatoes pic

Well for starters, kudos to both of you. Even with no picture from me, I'd say you guys pretty much nailed it.

Im leaning more toward iron deficiency in that it looks a bit more like that vs. herbicide. In part because the placement of the plants makes herbicide drift unlikely, and the leaves otherwise look pretty healthy. My thought too (and please correct me if this is wrong) is that if it is herbicide there isnt much for me to do other than give it time to heal itself.

So if it is iron, apparently a liquid supplement is the way to go. Im thinking I will pick one up at the nursery, and see if that works.

Following up...

Well it turns out what looked exactly like iron deficiency was actually the early stages of herbicide damage. Turns out a well intentioned, but uninformed neighbor sprayed some round up WAY too close.

Going forward, from what I have read, there is nothing you can do for the tomato plants other than "wait it out". That said, should I cut off the damaged parts, or leave them in place?

does it look like this?

{{gwi:86738}}

That is glyphosate damage, roundup. Only thing you can do it hope and pray. are there farm fields around you?