This page describes rational for upgrading to version 5 of Mysql.

Change log for 5.0.x can be found here....
dev.mysql.com/doc/refman/5.0/en/news-5-0-x.html

Upgrade Rational:

1) 4.1.x will be deprecated very shortly - bugs no longer pursued.
2)Current release is 5.1 - 5.0. is one still major release behind with a shorten shelf life
3) 5.0 is in production mode at PDSF and RCF there are compatibility issues.

Issues with Client:

1) the only comment I would have that native
SL3 mysql version includes version 4 of mysql client (4.0.26 to
be exact) aka the same than under SL3.0.5.  SL5 (much later in the
game) 4.1.11 so again from the V4 series.

2) Possibly, you will have to deal with special patching procedure
on nodes where you deploy V5 as it will unlikely be part of the
baseline erratas for V4 ... but that is probably a detail [as mysql
release the patches too]

3) There seem to be a few issues with the clients which need
upgrading (perl DBI being one).

--This issue is with the 4.1 client accessing the 5.0 servers...(Pavel trying to access rcf) I was not able to reproduce

The question is compatibility with
SL native distribution versions ... It would be an issue if our
PHP tools would break ... A full evaluation welcomed (with caveats).
Do you think you could get a PHP view of things?
--Tested and our PHP works fine with 5.0

4)  With regard to prepared statements....

- client MySQL libs version 5.x: LIMIT ? would pass a quoted
string.
- client MySQL libs version 4.x: LIMIT ? would pass an integer
and ensure type SQL_INTEGER in bind

Additional information

+ server version 5.x: would allow LIMIT ? and treated differently
than in version 4.x (hence the client version behavior difference)
+ client version 4 / server version 4: all is fine
+ INSERT NOW()+0 used to work but now fails with MySQL version 5 and up

Evaluations:

So far I have tested the 4 client with 5 server and 5 client with 5 server - with regard to PHP, perl

and command line (script) access.  Modulo the prepared statement issue stated above (limit ?) tests are successful and are in fact in production i.e., DRUPAL is php accessing a 5 server and the file catalog on heston is perl from a 4 client accessing a 5 server.