I recently faced this at work, and figured that sharing the problem and the options might be valuable. 

Here's the situation
My parents went away on a week's vacation and
They left the keys to the brand new Porsche...

Ok, that's enough of that.  Sorry for the interruption.  The real situation is that I had web application where the client data was in the top level site collection and a single sub-site.  I needed to copy this content to a new web application, and I wanted the destination site collection to be a host header-based site collection.  In general, I know that the following options are available for moving site collections:

Backup and restore. 

This option gives us the ability to capture the database and non-database configuration and content.  This does not backup or restore shared services configurations for the site, including Excel Services and Search configurations.  This can be done through the GUI, but the command line enables scripting...

Stsadm –o backup –url <SiteURL> -filename <FileName>

Stsadm –o restore –url <restoretoURL> -filename <filetorestorefrom>

Export and Import

Allows the export and import of site and subsite data.  The major difference between restore and import is that import allows you to merge data to an existing site where restore basically requires a blank site collection.  Another difference between backing up a site and exporting a site seems to be the -versions switch which allows you to specify how versioned files are brought over to the new site collection (you can trim DB size by dumping all but the last versions of files)

Stsadm -o export -url <SiteURL> -filename <cabfilename>

Stsadm -o import -url <SiteURL> -filename <filetoimport>

Content Publishing

Content publishing takes advantage of the MOSS Web Content Management capabilities which replaced Microsoft's Content Management Server 2002.  The end result is essentially a site collection photocopy capability which is defined in the UI in Central Administration's Operations tab.  Your mileage may vary with this option as I've had trouble with it before.

I hope this has been a useful summation of some site collection management capabilities.  I know this is not an exhaustive list and that there are plenty of options here, including 3rd party tools like ECHO Technology's SiteManager and more.