It looks like the Cortex M3 maxes out at 128KB right now but there are plans for 512KB +.(Page 3)
Acording to Microsoft, .NET Micro requires 250-520Kb.
So it looks like it will be awhile before this is a option but it would be great to have some managed code for LCD touch screens and keypads.
.Net Micro is a free download and works with Visual studio which is not free.