I've reported this as an issue to Salesforce support, but in our instance it is very easy for a user to bypass all validation rules against an Account.  I have not tried it with other objects yet.

1. Go to an object that has a lookup to an Account (e.g. a Task would do it)
2. Create a new object of this type.
3. Click the lookup button to pick the related Account
4. Click New, then simply fill in the name of the new account and hit save.

 Hey presto, you have saved the Account while bypassing all the validation rules.