I just shot some footage yesterday with my HD100U in DV mode. When I started working on it today I noticed a blocky grid type look to most of the footage. It's hard to see on still images, but here's a good example:

Full size:
http://www.famousdavispro.com/images/badhat.jpg

Detail:
http://www.famousdavispro.com/images/badhatdetail.jpg

Now it's only more noticeable on the red hat, but it appears to be on all the footage, regardless of subject. Is this a problem with the camera, the lens, the tape stock (Sony DVM60), or what?

It's a problem with shooting DV, and even more a problem with you over saturating the colors in DV.

4:1:1 chroma subsampling is used with DV. It basically means that your luminance is 720x480, but your chroma sampling is 180 x 480. (Can someone please check my math.)

Tim is right and his math is good.

And so is Perrone.

I never shoot in DV, unless the client wants DV tapes to take with him at the location.
I prefer to shoot in HD, Edit in HD and only after the final edit, only then to convert to what is needed to preserve the highest quality at the end.
Computers now-a-days can handle the HD pretty good as if you are editing DV years ago on a Pentium.

That doesn't look like normal DV artifacting to me. It looks much worse. I don't know this camera so it is possible that it is not optimized for SD. That is the case for example with the HVX200 which has awful stair step aliasing in SD, much worse than a DVX100.
Whatever the cause, it is not acceptable DV in my opinion though.

That's actually pretty normal looking DV encoding for an edge of pure red or pure blue. It would look the same with the DVX100.