Negotiating PhD stipend
I must say that I am surprised with the answers here.

I am willing to bet a fair amount of money that you will not get a raise from your institution. And I can think of a million reasons. Here are a few big ones.

A part of my reasoning stems from my personal experience, where I had multiple offers to top-tier US universities, and also from a slightly lower level US university, which offered me a salary that was almost 10k higher. When I took this offer to other top-tier US universities, they shrugged and said, "Oh well. You can't have everything." Also, in my experience, the top tier universities offered similar amounts that were not worth negotiating.

Another reason is that grad students are very replaceable. You may have some relevant experience, but they can easily fill your seat with someone else (and never know what they're missing out on, because you haven't demonstrated any of your academic and research abilities yet) so it's not really worth it to the admission committee to try and negotiate your salary.

Also, note that you may not have been admitted without your relevant work experience, so you may have already "cashed out" from your experience. 

Then there's the fact that a lot of grad students are admitted each year (as opposed to a couple of professors/postdocs). If the administration tried to negotiate salaries with 20+ graduate students a year, it would be a nightmare, not to mention that it would seriously hurt the morale of the student population to compare salaries and realize that there is some sort of a ranking amongst the grad students. Being in grad school is already damaging to your ego; can you imagine the student next to you getting paid a lot higher salary, and realize that you're not as wanted as her?

Anyway, I have never heard of grad students negotiating higher pay (unless you count getting federal funding, or research support from your advisor, or extra summer teaching etc.), and combined with the fact that you've already accepted your offer, I really don't see your institution increasing your salary. In fact, I wouldn't even try, because you're already committed to going there, and the people will remember you for trying something like that (OK, maybe it wouldn't hurt to ask once, but I wouldn't try very hard).

That being said, I am aware of a few cases where the grad students had two-body problems, and the institution was willing to accommodate that, but only in the cases where at least one of the two applicants were stellar.