Obviously, a lot of people are having trouble with the removal of many implicit casts in PostgreSQL 8.3. While this will lead to more robust applications in the future, it will prevent many people from moving to 8.3 altogether at the moment. The problem why you can't simply make the casts implicit again is that in a somewhat unrelated development, 8.3 will generate a cast from and to text automatically (well, implicitly) for any data type, which is why most of the casts in question have been removed from the pg_cast catalog altogether and you can't simply run an UPDATE command to put them back the way you want.
I have used a shell script to regenerate the removed casts including the required cast functions. The result is pg83-implicit-casts.sql. I have tested this against some of the recent "bug reports" I found on the mailing lists, so it should work for some people at least. If someone wants to use this, I suggest you only add the casts you really need, not the whole file.
(Note: This blog entry was recovered after a server crash and does not include any of the original comments. Those comments contained additional important insights about the restoration of the casts, which have unfortunately been lost now. The gist was, only restore the casts you need, not all of them.)