I like the thought of kitting and selling.
Frankly, making our own PCBs for sale is probably not a good idea. Plating involves some toxic chemicals, and while the sodium persulfate used for etchant isn't particularly toxic on its own, once it's taken up a load of copper, municipalities won't thank us for running it down the toilet. Silkscreening and soldermasking are something I've never event tried and frankly, I wouldn't want to, when you consider that even in smallish quantities, an Arduino-sized board, two layer, would probably run less than $15. If you bump it up to, say, 500 pieces, a two-layer board that size would be less than $5.
That said, when the day comes that we have a shop, I'd like to see a basic PCB fab setup there- an ultraviolet lightbox, a small stock of UV sensitized PCB stock, and a circulation/heating tank for etchant. I'd also like to see a toaster over for reflow soldering surface mount assemblies. This just for onesy-twosy boards, of course.
I mentioned some shields I'd like to see: an accelerometer/gyroscope shield, for one. I like Ladyada's motor shield, but it seems to be geared for larger motors- I'm not sure about adapting it to the smaller, 1.5-3V motors that come with the Tamiya kits. A port expansion shield would be nice, as would an I/O optical isolation shield. An LCD driver shield would be nice- something that lets you control a standard 44780-drive LCD with just two or three Arduino pins, and includes an inverter for EL backlit displays and a driver for the LED backlit kind, and possibly even a negative bias voltage option for the high-contrast outdoor types. Perhaps some sort of Arduino-to-PC interface shield- converts activity on the Arduino into keyboard/mouse signals on the PC. Machine vision kit? Maybe out of the range of processing power for an Arduino. A really simple 49MHz RC shield. An infrared remote control receiver shield, and maybe one that can play back IR signals. A relay kit that can switch fairly beefy AC appliance loads.
Also, I think it'd be cool if we could market our own Arduino boards. For that to work, we'd have to somehow value-add them: either sell them cheaper or add some really cool feature- programmed via IR might be fun. I don't recall seeing any kit-sold Arduinos out there- by letting hobbyists solder up their own boards we could cut cost and be a cheaper source. As has been mentioned before, many hobby makers have more time than money.
As was also mentioned, providing a complete package for beginners so they don't have to stress out over part selection. I'd provide two options: buy the kit, or just the PCB and we provide a free list of parts to buy- preferably with Digikey/Mouser part numbers to reduce ambiguity. Getting assemblies made adds to the cost but I've never priced it out. I've had good experiences having boards made by the Advanced Circuits folks in Colorado; they partner with a company call Advanced Assembly, who specialize in small board runs.
Wow, that message got long.