No, it is not acceptable. The web server should only deliver what the user agent tells the server to deliver. This is what the user agent does, even though it has changed since the origin, it is still valid and a cornerstone of web technologies.
"In HTTP, the User-Agent string is often used for content negotiation, where the origin server selects suitable content or operating parameters for the response. For example, the User-Agent string might be used by a web server to choose variants based on the known capabilities of a particular version of client software." - Wikipedia: User Agent
On Windows Phone 7, Internet Explorer delivers exactly what you expect – the full version of the site. And I find this site particular well designed where you have the option directly on the site itself – instead of having to tweak your browser settings. At the bottom of this site – there is a link (in mobile view) to access the full site (read desktop version). Pushing the link – you get exactly what you expect: the full site. This is caring and understanding user needs.
There are actually two options here:
- Follow the user agent in every step
- Implement a “full site” option on the mobile view