I'm extremely happy with my 13" MacBook Air and its 7 hour battery life. Chances are, I'll be tired of looking at my laptop before I'll encounter a situation where I run out of juice, and I have my iPad to tether, even though not technically allowed...
There's a Trekstor Portable 3G/HSPA Wifi router (a Rebranded Huawei E5) on its way to me right now. Lack of built-in 3G in the Macbook Air is a little annoying - I'd prefer it over the SD card slot - but this little router should do the trick. Dragging an iPad around together with an Air just for the 3G seems a little over the top. :-) An iPhone 3GS or 4 with bluetooth tethering would be OK I suppose; my 3G's modem is annoyingly slow (384kbit/s) due to lack os HSPA support.
