If you have a solid colour background, you can erase the background all at once. Open Photoshop (I use Creative Suite 4 and 5). Open the image choose SELECT from the menu bar, and then choose ALL. Copy the image (press CTRL-c) and open a new document, making sure that the "Background contents" field says: TRANSPARENT. Paste your image here (CTRL-v). Now to erase the background: from the menu bar, choose SELECT > COLOR RANGE then use the eyedropper tool to suck up the background color, then when the background is selected, hit delete and the background is gone.
Remember to save the file as a PNG or a GIF or TIFF because a JPG does not recognize transparent backgrounds - it just puts a white background back on the image.
Here, I took the white background off of my logo (kellyokelly.com) and put on an orange background. It may have to be done more than once to get it totally clean.