No issue comes to mind. Hopefully others will chime in if there are any known issues. Regardless, I'd recommend testing it under FileMaker 11 to be sure. If you haven't purchased a license yet you can download a 30 day trial.
The file format has not changed so there's no formal conversion that takes place. You just open your existing file with 11 and now have the option of using the features added since 8 was released.
Thus, you aren't likely to encounter many issues--if any, but testing your file yourself to be sure is always a good idea.