Re:Foundation 2010 site collection Administrator in site collection creation
HC creates OU per domain bases so if a webadmin have 2 domains i.e
Then in active directory HC will creates 2 OU with domain name.
In your case if a webadmin have sharepoint site and mail domain name are same then your are correct HC will add SP and exchange accounts in a same OU.
There was also a suggestion to allow user to chose OU creation and name at the time of exchange domain creation. This suggestion already under development and will be included in next HC8 installer.
Using new integration webadmin can chose unique OU as well as reseller will be able to share GAL segregation for two domains under different host (only reseller will allow to do that)