As others have pointed out, the Transformer does require a proprietary cable for charging. There are a couple of drawbacks to this.
- The supplied cable is not very long. My Nexus S phone came with a really long micro-USB cable and it's great, because I can plug the phone in and put it up on a high shelf out of reach of the kids (who know not to touch the wires).
- The cable is a proprietary plug on one end, and standard USB device plug on the other end. But don't be fooled: it won't charge from the PC. The Asus adapter (which is AC-USB power) detects the transformer and negotiates a higher current/voltage. The Transformer won't charge in a standard USB plug. You can, however, use the Asus adapter to charge normal USB devices without problems.
I am not sure why Asus went with a proprietary cable for charging. I suspect it's because that port is meant to be a multipurpose port for the dock, charging, and accessories, and that using a proprietary port makes all that much easier. However there aren't really any accessories for it and anyway they could have included a micro-usb port along with the dock/accessory port.
In any case, you would still need the wall plug adapter because of the higher power requirements for charging this device.