I know that storage is available via Isolated Storage, but I'm not sure I've seen anywhere how much actual storage I have access to for my app. Anyone know?
MS says a minimum of 8GB but devices may have more installed
http://msdn.microsoft.com/en-us/library/ff637514(v=VS.92).aspx
There are no limits for a single app
[EDIT] There IS a limit for a single app:
2 Gigs is the total size your app can grow to. http://www.imaginativeuniversal.com/blog/post/2010/10/04/20-50-90-400-and-2.aspx
Your application isn't limited to a certain amount of disk space or any quota (like on "full" Silverlight). You can, in theory, keep adding files/using disk space until the device runs out.
You can test for available space with IsolatedStorageFile.AvailableFreeSpace.
You should be careful about using all available space as it will impact other apps and general device performance. Hopefully the OS will stop anything horrible happening if you did use all free space but best to try and avoid being the cause of this.
There are some limits on the number of fies and directories you can have but you're very unlikely to hit these. Details at http://dotnetcatch.wordpress.com/2010/09/08/wp7-how-many-files-does-it-take-to-crash-isolatedstorage-more-than-you-think/
Just wanted to add that OS will start screaming when only 10% memory is left.