I've had this same problem myself twice in the last six months. If it is only a few specific apps having problems with updates not installing, then at some point in the past an update was being conducted but was interrupted. You now have a temp file in one of your phone's secure folders that is messing up your apps' update process.
Connect your phone to your computer via a USB cable and explore your phone's drive just like a computer folder. Look inside your phone's drive for a folder named ".android_secure". Inside that folder, you will see files with the extension of *.asec. You may see the names (or derivatives of the names) of your apps inside each of the file names. Look for one with "tmp" somewhere in the file name. This file (like your computer when you download patches and updates) is a temporary file that store all the downloading information until the installation process begins. If the data is corrupted or interrupted, then whichever apps were updating at that time will not install properly because the phone recognizes this file for those applications (which we now know is not complete).
Delete this file (if you are unsure if it's the right one, make sure to save it somewhere first, although it should be the only file with "tmp" in it) and you should be good to go. I actually just had my latest install problem cleared this morning by doing this. The six or seven apps giving me problems for the last couple of weeks installed their updates the next time with no problems.