Thank you for your post.
The file format for FileMaker Pro 7 through 10 is identical. FileMaker Server 9 merely hosts the files. Several FileMaker Pro 10 customers are accessing database files hosted by FileMaker Server 9, and you should not have a problem. Searching the forum, the few problems that have been reported include users being unable to see files hosted on the server, but those are easily fixable.
Keep in mind that if you add new FileMaker Pro 10 features, the FileMaker Pro 9 clients will still be able to use those files, but they will be unable to access those features.