Chris Johnson wrote a great post on how to accomodate a MySite host within a single web application. Normally, when you configure MOSS, during the SSP configuration most of us create a separate web application for the MySite host which means that we will have to host it on a different port. Thus when you click My Site from anywhere in the top level site, you get redirected to a link such as http://MyMossServer:8081/personal/username/default.aspx.
Obviously this is not the preferred approach and most organizations want a professionally done MOSS implementation without port numbers and fancy DNS tricks with host headers complicating things. Using the managed paths functionality of MOSS which is OOB, you can host MySites under the same web applicaiton as your main site collection.
Chris's methodology is essentially to create a managed path for MySite and Personal which will point to a MySiteHost site template which is added to the base site collection on the web application.