I have a Cannon Powershot SD1200 IS which is only 3 weeks old. I often get the message "Change Battery Pack" when turning camera on even though the battery is fully charged.

My IXUS 860 IS behaves the same. I found that if you start it in replay mode and then half press the shutter button it generally starts up OK.

Well, I haven't used a Canon PowerShot SD790 IS Digital Camera before, but I want to suggest this: before you start thinking of putting your camera apart, try to clean the battery terminals and the battery contacts of the camera with clean white paper, then reinsert the pack. Please be very careful when doing this so that you do not break or bend the contacts or terminals; they are usually delicate material.

Happened the same with mine. At the beginning I tough it was related with the sd card as when I removed the card the camera would turn ON. Later on I found it to be related with the camera contacts. After cleaning the battery contacts with alcohol and a cotton swab it started to work fine. SOLUTION: Clean battery gold contacts.

Try cleaning the battery contacts in the camera with a cotton swab lightly dampened with rubbing alcohol.

Here are some simple things to try first for a camera that won't power on. They won't work for every case, but they're worth a try:
http://camerarepair.blogspot.com/2007/11/simple-fix-for-dead-cameras-that-wont.html

If the lens is buzzing and followed by an "E18" or "lens error" message then the lens will need to be replaced. If no message appears you may just have flat batteries or even a single bad battery can cause this. Try getting some good quality disposable batteries specifically for digital cameras. If all is well with the new ones, invest in a new set of rechargeables.

Yes, rechargeable batteries go bad whether or not they're used, and are only meant to last for a year or two.

simple! with my Canon S90 had the same problem until I changed the SD card from LEXAR for a TOSHIBA SD card and the message " Change the Battery Pack"disappeared.

Some of The Canon Powershot A530 and A540 cameras are subject to an advisory recall. Seems that a small spring on the battery compartment door can short out the batteries. Suspect this of yours, in that you mention that your batteries aren't lasting long. If your camera is on the list, Canon should fix this for you for free, including free shipping both ways. This is regardless of your camera's warranty status. Please check the following link for more info:

http://camerarepair.blogspot.com/2007/12/canon-a530-a540-with-short-battery-life.html

I figured out the problem. My AA battery recharger was not making good contact, so although it said the batteries were recharged, this was not the case. After fixing this and getting full recharges, the problem went away.