Hi, there should be no problems at all with upgrading to PHP5. I have done it on a number of my own servers and I know a lot of hosts have already upgraded, with few-to-no reports of ListMail problems.
I do have one suggestion (assuming a Linux server): once you install PHP5 you will likely have a brand new /etc/php.ini file. I advise setting "safe_mode" and "output_buffering" to "Off" for best results. If the server is shared you may prefer to disable safe_mode for just your ListMail domain via the vhost (apache/httpd) configuration file.
Disabling safe_mode, in my experience, allows the ListMail PHP scripts to run beyond the normal/default 30 second script execution time limit.
Disabling "output_buffering" will allow ListMail's "live counters" to work, shown when sending email and importing users. This requirement should be gone in the future when counters are made JavaScript / AJAX-based.
Regards
