I bought an Ardweeny this weekend at Maker Faire, not realizing I would need a USB-to-TTL adapter to upload sketches to it. Before I go and buy one, is there a way I can upload sketches to it using the Duemilanove I already have?
Swapping chips back and forth seems a good way to damage pins. ArduinoISP sounds like what I want, but it seems to only be for putting the bootloader on new chips. Any suggestions?
Sounds like you could get by using a serial pass-through program on a Duemilanove. Just echo any characters coming in to the Ardweeny, and vice versa. Then connect the serial ports together (and ground connections too).
The trick is getting the Duemilanove to NOT reset itself when the IDE tells it to (you can cut a jumper to this effect) and resetting the Ardweeny at just the right time.
Frankly, those USB-to-TTL adapters are so handy in several situations I'd just buy one 
--
The Quick Shield: breakout all 28 pins to quick-connect terminals
this is not what i meant, but i comes close. I once lost the cable for my Sanguino and found a diy solution.. can't seem to find it now though it should be out there somewhere on the interwebs...
Look here: http://arduino.cc/en/Tutorial/ArduinoToBreadboard
example 2: "Uploading Using an Arduino Board"
but buying the adapter may be the better choice.
Thanks. I decided to opt for the serial cable.