Subfolders in data/ should be doable.
Are you talking about gigabytes of data?
If you are going to have files persist there for a long time (eg. you fetched a bunch of mp3 files for some reason), then it would be nice if your app would clean this cache out occasionally, or provide the user with a means of doing so via your app.
The reason I suggest this is because users don't have access to your app sandbox, so they may not be able to find out why they are running low on disk space and correct the problem. Only uninstalling your app will cause the sandbox data to be deleted.
Your other option would be to use an appropriate folder under shared/ somewhere (maybe shared/downloads/your_app_name). This way users would be free to clean up files on their own, however it has the downside that when your app is uninstalled, the files will remain and use up disk space.
The tmp/ folder in your sandbox can also be used to store large files, but it is cleaned out when your app exits.
Cheers,
Sean
