I often let my users decide which site template they want to use for their new site collection. In SharePoint 2007 i did this by creating a site collection using stsadm without specifying a site template. this caused the first user to have to select the site template to use. There was no possibility to create a site collection without specifying a site template through central admin.
Now in SP2010, the possibilty to not specify a site template has been built in to Central Admin. When creating a new site through Central Admin, notice that there is now an entry in the Custom tab, called ‘ < Select Template Later … >‘
Now when your user hits his/her new site collection, he/she will get redirected to the templatepick.aspx page and has to pick a site template.