It's against Adsense ToS so I wouldn't risk it, the one you saw is taking a big risk with their account. From Adsense Program Policies:
Publishers are encouraged to experiment with a variety of placements and ad formats. However, AdSense code may not be placed in inappropriate places such as pop-ups
, emails or software. Publishers must also adhere to the policies for each product used.
Google ads, search boxes or search results may not be
- Integrated into a software application of any kind, including toolbars.
- Displayed in pop-ups or pop-unders.
- Placed in emails, email programs, or chat programs.
- Obscured by elements on a page.
- Placed on any non-content-based page. (Does not apply to AdSense for search or mobile AdSense for search.)
- Placed on pages published specifically for the purpose of showing ads.
- Placed on pages whose content or URL could confuse users into thinking it is associated with Google due to the misuse of logos, trademarks or other brand features.
- Placed on, within or alongside other Google products or services in a manner that violates the policies of that product or service
If I can't be a good example, I'll just have to be a terrible warning...