I haven't really tested this on broadband vs mobile connection as load-balancing so I may be wrong. Actually based on your comments I trust you know enough of this, just blabbing on for the possible benefit of the larger community.

But the splitting does not need to be random, basically the kernel can be configured to use interface X until it's "bandwidth is reached" (you need make some educated guesses about the bandwidth) and then spill over to interface Y.

Or to divide the streams randomly so that 90% got to X and 10% to Y (but then we have the problem that you might get the slow interface even if the fast one had free bandwidth for you).

Of course this makes any sense whatsoever the situation is such that the place where the wifi connection is has (or allocates to you) really louse bandwidth and that your 3G provider has awesome coverage in the same place.

But this all a bit academical, doing all this is just too much work for the relatively small benefit (basically for every wifi network one would want to augment with 3g one would need to have different setup scripts and do a fair bit of experimenting regarding the best way to route the streams).

It will also totally mess your login sessions on sites that try to prevent session hijacking by tying the session to IP address.