Tetherberry and DM with IP Modem feature are not the same thing and do not work the same way.
TB installs an Ethernet adapter and creates a virtual netwok connection (like a Wireless or GbE adapter). It also works by creating a tunnel of sorts - which then masks your data connection to the carrier. TB cannot run unless this tunnel is created - which is why you need to run the TB program on your BB and on your Desktop.
DM or ATT Communications manager both use the BB as a modem using the device that gets installed on a Com port when you install either of them. They actually use the carrier's APN (like ISP.CINGULAR) and therefore if you do not have a tethering plan, you could get charged by the carrier. This incidentally is the same if you are using Bluetooth tethering.
When you use TB, you do not see a "Modem mode enabled" line on your BB, whereas when you use DM or ATT Communications manager, it says "Modem mode enabled" i.e. the BB knows that it is being used as a modem. In fact on the Bold (9000), modem mode used to disable Blackberry service on the BB. On the 9700 it keeps the service active even in modem mode.
9700 on 6.0.0.something, BES 4.1 on Exchange, BIS (AT&T)
Ex 8700, 8800, 8100, 8310, 9000