There are some issues that you could be running into, but first off I should say that there is a good chance that this is a bad idea. If your installer is ever run with root permissions (such as by a admin over Apple Remote Desktop) the you are going to wind up launching an application as root, which can lead to bad things, this is probably the reason that Apple has not included a step to startup applications only to open files (this probably uses the "open" command, which is a little safer).
You are going to want to make sure that you don't run this if you are installing as the root user. So you are probably going to want to make another install item that is hidden, and checks to see if the UID is 0, and if so skips running. Then you are going to need to manually script this in. Right now there is not much documentation for this for the 10.5 version... in time that will come.
