If you look at the Debian Repos, the latest versions of Banshee that you can get are 1.61 (Testing/Sid) and 1.74 (Experimental). Thus, if you want 1.8, your only option is to either compile it from source or use a Ubuntu .deb. You should always be cautious, however, in mixing Debian and Ubuntu packages.
To compile from source the Debian way, you might take a look at the following:HowTo Build a Package from Source the Smart Way:http://forums.debian.net/viewtopic.php?f=16&t=38976How to install when there's no DEB available?:http://forums.debian.net/viewtopic.php?p=143816#p143816
Those threads also point out that if you find a .rpm of 1.8, you could use Alien to convert it into a .deb.
I played around a little and tried to create a deb using the Banshee source code using the methods described above, but I had no luck.