You don't have to poll if you use a FileSystemWatcher
inside your Windows service for real-time notifications.
http://msdn.microsoft.com/en-us/library/system.io.filesystemwatcher.aspx
However, if your requirement is 100% accuracy, you will need to build in some sort of tracking/checksum mechanism to make sure that every document was 1) detected and 2) successfully moved to SharePoint.
You may want to have your service check the delta every time it starts up, and then subsequently only respond to FileSystemWatcher
events.
EDIT: Per Tony's question below, here are some additional thoughts on getting files to SharePoint.
First, try a simple test.
1) Copy the URL of a document library within the BPOS SharePoint site. Make sure you're on a machine that has the Office Online sign in app on it.
2) Open Notepad. Type some random text.
3) Click on File -> Save As.
4) Paste the URL.
5) Attempt to save the file.
This works great on "regular" SharePoint (done it many times). If this works with BPOS, it opens up several options.