AKA "I'm too cheap to buy a proper router that supports 3g out of the box".
I like Tomato, so...
There are several alternatives
The DANtes one is rather hard to download given you must register on the Russian forum. I couldn't register as I failed to answer some of the questions (in Russian). There are some downloads of older versions here http://tomatousb.org/forum/t-254630 but as you see from the forum page here: http://sat-forum.su/index.php/topic,6629.30.html there are newer versions.
The DualWAN mod http://code.google.com/p/digiland/downloads/list doesn't support USB in the 4M version.... except for this one (Tomato Pandora 4M R1 3G 0490): http://code.google.com/p/digiland/downloads/detail?name=tomato-K26USB-1.27.0490-R1-3G.trx (which doesn't seem to be "Dual WAN", oh well...)
On the Pandora 4M one, the default password is "tomato", not "admin". FYI. Also, the interface is in Chinese. So, to save you some time using Google Translate:
Other firmwares that may support 3G:
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