I've recently hooked up my Arduino Mini ver 04 and USB Mini ver 03 - and I am finding out that when I use the power from the USB it does a few bad bad things. >:(
First It wont allow any code to be pushed to my Arduino Mini - To get around this I've had to unplug the USB and reset - once I plug back in I also have to remove the jumper from the USB mini - and this mostly works, occasionally I'll need to repeat that about 2 or 3 times before the code will actually push.
If I just plug in and click the serial monitor before I upload - I get lots of random characters spit out... I can then remove the jumpers from the USB cable and it usually stops...
Second if my code uses the SPI interface - if I am powering with the USB Mini - it causes erratic behavior and random bits to be thrown out. Using the SparkFun LED Matrix works perfect on my Diecimila boards - but once on the Mini it freaks out. The only way around this is to power my Arduino Mini from the old Diecimila board insted of the USB Mini... Once I do that the SPI code works and everything is happy - except now I have a big ole Diecimila hanging off of a Mini... Not sure what is the problem? Bad USB Mini?