In SharePoint 2007 it was possible to create a site template (stp file) of a site and use that template to create new sites. When you downloaded that stp file and added it to the global template gallery by using the stsadm command addtemplate, you were able to create new site collections based on this template.
With SharePoint 2010, this mechanism has changed a bit. Site templates are no longer stp files, but when creating a template of a site SharePoint creates a Sandboxed solution which is placed in the sandboxed solutions gallery.
A possible solution
On his site Todd Klindt explains how to use these solutions to create new site collections based on this template.
Unfortunately this method has the downside that it requires manual actions, each time you would like to use the template.
Another solution is to add the solution to the Farm Solutions gallery. This has the advantage that you don't have to upload the solution each time. But the downside is that when adding the solution, the template does not become available in the template selection, but is added as a feature to all site collections. During site collection creation you therefore still need to activate this feature before you can use the template.
Then how to solve this issue farm wide......simply by making a small change to the solution. The template feature in the solution is scoped to a site collection by default and therefore uploads the template to the site collection template gallery. If you change the scope of the feature to Farm and then activate that Farm feature, the template is globally deployed and available for selection during site collection creation!