What would you say if a developer wanted to implement a sql2008 dev environment, but we were still forced to use a sql2000 test and sql2000 production environment?
Would there be anything wrong with using sql2008 on a dev server? Of course you'd need to know what functionality you couldn't use, so you didn't have problems migrating your work from the sql2008 servers to sql2000.
I'd strongly avoid developing on a different local version than the dev/qa/prod environments. Most of the time nothing will happen, but when it does it can take forever to track down the issue. Not only that, you may never be able to replicate it locally since you have a different environment.