There is an excellent discussion going on here over at Absolute Write's paying market forum that reads like a primer of everything a writer should check out and question about open anthology calls. I'm particularly impressed by how the writers on the site are responding to the post (and if you do go over and join in, please be as civilized about it as they are.) Protecting each other by flagging and (when possible) discussing prohibitive terms involved with any writing job is one of the most valuable aspects of being members of the online writing community.
In the past I've done flat-fee writing for hire, which in my case involved turning a concept into novels for one client. Like most writers I much prefer to do my own thing, but at the time I needed the income, and when bills pile up you often can't afford to be choosy. It turned out to be a neat challenge involving an interesting type of writing, plus I got to use some personal knowledge I don't usually have the chance to employ in my own writing. I worked with some great editors and was suitably compensated for my time and efforts. All in all it was a decent experience and I'm glad I did it; if the right project came along I'd certainly do it again.
That said, before I signed the contract for that particular WFH project I made sure I understood every aspect of what I was selling to the client: all rights, including the copyright, in exchange for a flat fee per novel plus a standard percentage of the royalties. Once the books were finished they belonged to the client, not me; from my POV they might as well have been written by someone else. Which is how I still think of them, actually -- as the client's books, not mine.
Bottom line, be sure and read all the terms involved with any offer or writing job you're considering, and be sure you're clear on what you're selling to the publisher or client. If any clause is unclear, ask for some clarification. If any of the terms leaves you feeling uncomfortable, worried or otherwise troubled, don't accept it. You can always find another job -- once you sell off your rights, they're gone forever.