You're getting «HRESULT: 0x80070005 (E_ACCESSDENIED)» when you try to create subwebs using a custom site definition programmatically?

For a quick work-around, just add this to your Assembly.info.cs file, re-compile and re-deploy:

using System.Security;

...

[assembly:AllowPartiallyTrustedCallers]

Signing and making your code trusted should also work, but if you're on a tight spot, this code will probably get you some air to breath

Hope this helps someone out there