I want to create a site definition based on the my sites template (i.e. so that when I provision my sites, the site collections created include my customisations as in the vs2010 project.
How can I achieve this with site definitions in vs2010?
You can't change the Site Definition SharePoint uses when it creates the Personal Sites. If you want to do any customisations you should implement these as Features (that would be my recommendation anyway) and apply them using Feature Stapling.
Remember that if you wan't to change web parts front page, this isn't created until after stapled features are activated, so you need to do that through a delegate control which can make the changes first time it's shown.