You can do your development in 11 and the file format (.fp7) is backward compatible to 10. With this CAVEAT. Any features that were introduced in 11 will not work in 10, of course, and you may get unexpected results. So check the Knowledge Base on changes from 10-11.
and be sure to drill down. For example, there were some new script triggers introduced in 11 as well as some behavior changes. You will want to be SURE you do not use any of the 11 only features in your development.
You should be fine as karen says just don't use any 11 specific commands of functions that are not in 10 but the file format is the same. I am been doing this for years without any problems.
Another significant caveat: Some functions were changed. For example, Get ( PrivilegeSetName ) in 10 became Get ( AccountPrivilegeSetName ) and Get ( CurrentPrivilegeSetName ) (can't remember which is the default). NEITHER is backward compatible. So if you are using any of the functions that got changed, you''ll need to insert them using a copy of 10. They'll show up correctly in 11 with their new name, but if you use the new name in 11 and then try to run it in 10, it'll break.
Edit: I just checked. Get ( PrivilegeSetName ) became Get ( CurrentPrivilegeSetName ). Get ( AccountPrivilegeSetName ) was added. So if you use Get ( PrivilegeSetName ) in 10, it'll convert to Get ( CurrentPrivilegeSetName ) in 11 - but not the other way around. Just one example.
Thanks Karen, Lemmtech and Mike. This is exactly the kind of information I was looking for.