If you are using PHP 4 and using some of my plugins, the error message “Warning: cannot yet handle MBCS in html_entity_decode” will get displayed on your page. This is not a bug in my plugin but a bug in PHP 4  itself.
My recommendation would definitely be asking you to upgrade to PHP 5. PHP group has announced the end of life for PHP 4 on the 13th July 2007.
Today it is exactly three years ago since PHP 5 has been released. In those three years it has seen many improvements over PHP 4. PHP 5 is fast, stable & production-ready and as PHP 6 is on the way, PHP 4 will be discontinued.
The PHP development team hereby announces that support for PHP 4 will continue until the end of this year only. After 2007-12-31 there will be no more releases of PHP 4.4. We will continue to make critical security fixes available on a case-by-case basis until 2008-08-08. Please use the rest of this year to make your application suitable to run on PHP 5.
As you can see, there would not be any more PHP 4 updates and security fixes will be available till 8th August 2008 which is only 2 more months. If you want to upgrade to PHP 5, this is the best time to do it.
If for some reasons you are not able to upgrade to PHP 5, the only thing you can do is to turn off error message display. You can refer to this thread  for more information. This is definitely not an elegant solution.
If you host supports PHP 5 but you are on PHP 4, you can take a look at the solution posted by Thomas Arie Setiawan .
Softwares like MediaWiki  which powered Wikipedia  requires your server to be running PHP 5, and vBulletin  has beginning asking customers to upgrade to PHP 5. It would be a matter of time before WordPress requires a minimum of PHP 5 to be running.