Hi Yellow,
An offer of a job is fantastic, well done. If you really want a postdoc position, and haven't been able to find one, you could take the scientific writing job but keep looking for postdoc positions at the same time. As far as I'm aware, it wouldn't be a good idea to do the writing thing for too long as you might be considered to have been away from research too long and more recent PhD graduates would be prefered, but if it's only for a year it shouldn't be a problem. You might even find you really like the writing and decide not to persue a postdoc position :-)
Good luck with it all