Besides tethering (I'm on 2gb/month), what are some reasons to JB the iphone 4?
What kind of apps are so amazing that you should do it?
iOS4 hasn't obsoleted any of the reasons for jailbreaking, with the possible exception for some people of Backgrounder and Winterboard. Even those two apps have only had a part of their functionality implemented by iOS4, so many people will still want the JB apps.
iOS4's multi-tasking functionality doesn't support every possible type of multi-tasking that people might want, and it also doesn't work for apps where the developer hasn't explicitly added support for it, so Backgrounder will still be useful for many people.
iOS4 now supports custom wallpapers, so if that's all someone was using Winterboard for, then they might not need it any more. However Winterboard supports much more extensive customization, so people who like customizing their phone's appearance will still want Winterboard.
Most other JB apps, such as iBlacklist, Terminal, SSH, SBSettings, 3G Unrestrictor, MyWi, PDANet, QuickScroll, etc, are still just as useful as before.
EDIT: I forgot about folders. I would say that iOS4's folders feature does obsolete the Categories JB app. That's the only one I can think of.