E.g. If I signed up to Myspace I would use Myspace@exampledomain.com
I have the mail server at "www.exampledomain.com" set to accept all emails under the domain so I can see if someone has passed on my details legitimately or via hacking.
Since I've started about 12 months ago I've not found any cross pollination which seems a good sign for the industry in general.
It also adds a layer of security as your sign-up email changes for different websites if you use the same password across several.