First I know you've said you've checked the card's slide switch, but just want to restate this first for others with the same problem who may not know of this switch.
Check to make sure that you didn't accidentally place the tiny locking slide switch on the side of your SD card in the "locked" downward position. The correct "unlocked" position should be slid up toward the direction of the connectors.
If you've tried this, and it's still showing locked, it's likely that the locking switch inside the camera's card slot is stuck. If you look inside the slot on the side of the slot where the SD card slide switch would be, you'll see a glint of gold color on the side. This is the camera's lock/unlock switch. The other glint's of gold that you may see at the far end of the slot are the reader's connectors (ignore those).
Somehow, the locking switch is making contact and has become stuck in the locked position. With a plastic toothpick, or other skinny NON-METALLIC item, tap against this switch to see if you can jar open the contact to its normal unlocked position. Also try blowing compressed air at it, as some gunk may have worked its way in there, jamming it closed. Occasionally, reinsert the SD card to see if the issue has been corrected.
If repeated tries fail, it may be time to consider opening the camera to force open this switch (not recommended unless you have some familiarity with electronics). In this case please see the following (xscrewdriver, hope you don't mind me referencing your blog):http://www.myfixtips.com/blog/digital-camera/how-to-fix-a-%e2%80%98card-locked%e2%80%99-problem-125.htm