Andy,

It is quite fine to attach things to a USB port in order to get them working. I believe that Raghu appears to intend to get his device working as a full USB device. One could conceivably work for many days with a circuit hooked up and drawing power but not yet ready to be functional enough to be able to respond to enumeration requests from the host.

The main thing to be careful about is to not short out or overload the 5V rail coming out of the computer. Some times a short like this can make the computer crash. Other times it will blow a fuse, trip a polyfuse or make a USB port power switch chip detect an overload and turn off the power.

Keep in mind that there are coutless devices out in the market that have a double ended host side USB cable that get power from two ports to support an attached DVD drive or 2.5inch HDD. Here is one such type of cable:



Then there is this type of cable that I've built several of that are specifically made to borrow some 5V power from a spare USB port:



Michael Karas