My colleague installed Exchange 2007 onto a new Windows 2003 server but decided against joining the server to our domain. Can anyone foresee any difficulties that I may encounter by connecting this server to our domain? Thanks in advance, Gareth.

What role did he install without domain membership? Edge transport? If so, an edge server should be off domain anyway.

EK7 Edge is the only role that you can install on a server not being member of a domain, all other roles must be installed on a server being member of a domain.