PHP 5.3 hasn’t even been released yet (yes, there’s an RC), so it’s a little early for widespread panic. Not that a notice would be the end of the world, of course.
To quote the php.net website:
Expect an RC3 in 2-3 weeks, although there will not be major changes so now is a good time to start the final testing of PHP 5.3.0 before it gets released, in order to find possible incompatibilities with your project.
In other words, the EE community also needs to test their product with the bleeding edge versions of PHP.
As far as the notice, the EE developers constantly claim that they are trying to make their product as fast as possible. The reason ereg is being deprecated because it is slower than preg_match. If that’s the case, why haven’t they switched?
The other issue is that no extensions will work now! This line on 120 of core.extensions.php
call_user_func_array(array(&$this, 'universal_call_extension'), $args);
causes the following warning:
Warning: Parameter 2 to Extensions::universal_call_extension() expected to be a reference, value given in core/core.extensions.php on line 119
This sounds pretty serious to me!