Read original post by Pratap Lakshman at The Windows Phone Developer Blog
Performance is a concern for all but the most trivial of applications, and in a constrained environment like the phone it is all the more so; performance issues directly manifest themselves in the user experience. Memory usage is one such source of performance issues that can degrade user experience, and the Windows Phone Marketplace enforces a technical certification requirement that an application must not exceed 90 MB of RAM usage, except on devices that have more than 256 MB of memory.