Start by disconnecting the probe pins one by one and see which one is causing troubles; revise your pin usage strategy.
He said above that he's not using any digital pins except one for the One Wire temp sensor, which should work just fine with the other hardware he's using. Meanwhile, he's got no decoupling caps on his circuit and no pullup resistors, both which CAN cause this sort of problem.
PS. there is no reason to pull up the I2C's SCL and SDA. Leave them alone. It will eventually make your system unstable.
Do you have something to back that up with? All anecdotal evidence I've seen (my own experimentation, forum posts on arduino forums, commercial implementations using I2C with arduinos) suggest that external pullups are a good idea. In addition, all the "official" documentation I've seen supports that position too, including the I2C-bus.org spec and the original Philips I2C spec, which call for pullups stronger than the internals activated by the Wire library:
http://www.i2c-bus.org/fileadmin/ftp/i2c_bus_specification_1995.pdf
http://www.i2c-bus.org/
Making statements like this:
Missing or wrong termination is the source of many field problems. As a rule of thumb, the higher the termination the better the signals.
Also, look at page 43 of this document, NXP's current I2C spec:
http://www.nxp.com/documents/user_manual/UM10204.pdf
It shows that the ATMega's internal pullups (50k ohms) are essentially too weak for even the best-case circuits.
If you've got experience or documentation to back up your position, please share, as I'm always eager to learn.
Also, different subject, just saw this from earlier:
The atmega chips run at 5V so plugging that plug-pack in directly to the Vin pin would be catastrophic.. If you gave nice clean 5v on the Vin pin would be ideal.
The Vin pin on the Duemilanove goes to the
input side of the onboard regulator - it's essentially the same thing as the power jack on the edge of the board, so putting +12v on Vin is the same as plugging +12v into the jack, and is perfectly safe. The only real difference is that the Vin pin is "downstream" of the reverse protection diode, so you want to be sure you don't connect power to the Vin pin backwards. For reference, the Duemilanove schematic:
http://arduino.cc/en/uploads/Main/arduino-duemilanove-schematic.pdf
What if a few other vendors put pullups on their I2C slave breakout boards, and I put them all on my bus? Having too-strong termination certainly CAN cause problems (it'll exceed the current limit on the bus lines!)
