Nope, NBA 2K1 and NFL 2K1 do not work with it. But, NFL 2K2 and NBA 2K2 are supposed to ship this year (yes, even after all the hardware announcements... they are still producing software), and they will support broadband. Also, NHL 2K2, Virtua Tennis 2K2, and World Series Baseball 2K2 will also support broadband. That's why I bought the Dreamcast.. the sports games are top notch.
Quake 3 Arena supports the broadband adapter. It works like a champ, except for the fact that I get mercilessly fragged by obnoxious punks who hang out chatting on Quake servers. (I'm waiting for the DC Unreal Tournament.. I can hold my own in UT.
The current Sega Web Browser also does not support broadband. The 3.0 version of the browser should be out soon, and include broadband support. You can get the latest version at
http://newbrowser.sega.com/ But, I think that currently they are shipping version 2.0 or 2.6.
Phantasy Star Online also supports broadband. But, it appears to need configuration to be completed in your WWW browser first.. and since I don't have that, I can't configure it. But, it does recognize the ethernet & says it will use it to connect, but then gives an error about the config.