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I upgraded from Redhat 5.2 to Redhat 6.0 amd all went well except for an nnoying problem with the permissions on /dev/ttyS0. The scripts for connecting via PPP (ppp-on, etc as detailed in the PPP-HOWTO) use pppd with setuid root so that any user may connect. The problem is that each time pppd is run and connects it sets the permissions on /dev/ttyS0 to rw-r-r--. No user can subsequently connect. No matter how many times I reset the permissions, the problem reoccurs.
That's a feature, not a bug. The comment above the code that does that says: "Do the equivalent of `mesg n' to stop broadcast messages." If you want to change this behaviour, you may be able to put a chmod command in your ip-up script; otherwise you will have to recompile pppd. Alternatively, you can set up pppd the Red Hat way as documented in our manual, and select the option that lets any user start and stop that ppp device.