I agree with Remi in that the php53 packages are a poor half-measure. A lot of secondary packages are not built for it and it will be more difficult to move forward in the future.
Red Hat is providing the same packages that they are providing for PHP 5.1. Red Hat does not control what third party repositories do with this. Although they might have set this up in a way that would have made it easier for third party repositories, I'm not sure.
I think RedHat will have to rethink some of the premises of EL in general. 7 Years is a long time to lock into a particular package and, in reality, PHP 5.1 is long past EOL.
You're not forced to stick with the same OS version for 7 years. The point is exactly that you can
stick with the same stack for 7 years if you want to. That's the value that Red Hat provides. They make sure that the software is maintained, even when the original maintainers have declared it EOL. That's actually pretty awesome.
It's also important to note that PHP 5 is an increasingly mature application. The pain of 5.3 to 5.4 should be very minor now (even MySQL upgrades are now pretty painless) and I can't see 5.4 breaking anything 5.3 compatible.
That's not the idea I got from reading through the 5.4 beta changelogs. They'll be removing functions and settings (safe_mode, magic quotes, etc.), so it definitely won't be a drop-in upgrade for everyone.