Recently there have been some rumours about Windows Phone Tango and how it will be a new version of Windows Phone for lower phones with 256mb ram (see http://www.zdnet.com/blog/microsoft/as-more-windows-phone-tango-info-leaks-questions-resurface/11907 ). This reminded me of a MIX11 session back in April 2011 by Jeff Wilcox where he said that
90mb is the magic number
for application memory usage. It would seem that Microsoft has been trying to prep the Windows Phone eco system to use less RAM in their apps presumably to allow them to run on Tango devices. Obviously this is to help avoid the issue of not being able to run them as shown in some of these leaked screen shots Tango OS.
Jump to 19m 25sec on the video where he talks about memory requirements.
Note: Jeff Wilcox is the author of the very popular 4th & Mayor app has just moved from the Windows Phone to the Windows Azure team at Microsoft