A lot of applications check if the current connection is 3g, and restrict background data transfers, on the assumption that 3g data plans are limited and expensive.
With a 3g-wifi bridge though this distinction blurs, and I end up with packagekit happily downloading hundreds of MBs in the background and draining my data plan every day. Not good.

Android as a feature that marks a wifi connection as "mobile hotspot", which restricts background data and warns for large transfers. NetworkManager could have a similar tag for wi-fi connections (or really any connection), and apps would look for it.

This looks superficially similar to bug 723906, but in practice it's different. Here the tag would be applied by the user, knowing his data plan and whether large downloads are appropriate or not.