Yes I do it if I think that an image has been refused unfairly and nearly every  time it has been approved on the second attempt.  If I agree with them then I fix it before re-oploading or give up on it if I think it isn't worth the effort.

A lot of times I think the first review is right on, especially with agencies like DT. On the other hand, I've had some rejections that make me wonder if the reviewer is very familiar with what he/she is doing at all.

Some examples of the rejections I've seen:

"Overfiltered" for an shot of some cheetos against a black background. The image was a raw (as in uncooked, not the the file format) and had only the tiniest (and I do mean tiny) curve adjustment. My thought was "Cheetos are bright orange. What color did you expect them to be?"

"Artifacting when viewed at full size" on a shot of a red bell pepper against a black background. I went back and checked the image and did not see artifacting until I had hit 200%, and then it was only minimal. My thought was either the reviewer was looking at it at greater than 100% or he/she has a really crappy monitor. Of course, maybe it is my crappy 19" .22 pitch monitor. 

And my favorite rejection was my recent application to CanStockPhoto. Images that were accepted at 123rf, DT, and FT were rejected as not stock material.

Of course, I'm pretty new at this and still learning, but I am, hopefully, improving. Of course, I've been shooting with a mid-to-high end P&S, but last weekend I went out and spent half my tax return on a Nikon D40X which should help rid me of the noise issues and hopefully improve my acceptance ratio, especially on DT. It should also help me get on the big three where my old camera just wasn't quite up to par.

Ok, back to the original intent: I have, on rare occasions, disagreed with the review enough to resubmit a photo and every time it has been accepted. I only do it when I am absolutely, positively sure the reviewer made a mistake and after the initial "that's bullcrap" reaction has calmed down to a "okay, let's see who's right" attitude. Hey, they are human and can hit a wrong button on occasion. And let's not forget about how many images each reviewer must look at each and every day.

But I have learned enough to know that some of my images subjectwise and stylewise are on the fringe of stock and really do fall to the whims of whether the reviewer sees it as stock material or not. In these cases, I simply give the reviewer a one fingered salute and continue on. 

We now return you to your regularly scheduled programming.

I don't think I have ever reloaded an image that was rejected.  I know that the reviwers make mistakes, but I just don't bother.  I have sent an email to support on certain sites and had a decision overturned.  The difference is that the problem is brought to someone's attention.  If a reviewer gets enough complaints about him or her then he or she will either be released or have to fix themselves, but if we just keep reuploading the images to get around that sorry reviewer then the problem persists.   

I have re submitted images that have been rejected - maybe after a few months and have got them accepted.

Mostly because I believed in the image- and yes, more than likely it will come before a different reviewer.