I'm able to reproduce this on SS3.1.1 and SS3.1.2 so far, on linux/apache2 and on a mac running MAMP.
I've tried php 5.3 and php 5.4, with APC enabled & disabled.
1. pick a getCMSFields (Page or modeladmin) without a GridField
2. insert echo(memory_get_usage());
3. reload the according CMS interface several times, memory usage stays consistent (or bounces between 2 vals)
4. add a basic GridField
5. repeat step 3, the memory usage consistently climbs.
In my case, it's climbing by an increased amount on each load (9k, 12k, 13k).
With this happening on every interface with one or more gridfields, memory will run out quickly for a heavily used CMS.
Any solutions? Can you reproduce this?