Here is how I understand TetherBerry
Tetherberry: it's a program that runs off your blackberry so it mimics a regular blackberry program and some carriers do not see this as tethering. That way you can avoid the annoying extra charges that come from tethering YMMV.
Regular connection (IP from DM): most carriers can charge for tethering and will mostlikely get dinged if you are doing it this way
Also if you live in Canada. A tetherberry connection will be seen (by websites) as an US connection so you can surf all the last.fm, pandora, HULU! all you want :D