A few points I would like to make:

1) There is very little chance of them closing their doors in the near future as they raised a large series A and have great growth.

2) Having personally talked with Tikhon on the subject and it's clear that they plan to make this a stable platform for the longhaul. I would not hesitate to build a project on top of Parse they are a great group and would not leave their users hanging.

3) If you're still hesitant keep in mind you can still keep mission critical stuff on your own Servers/APIs and use Parse for Push Notifications/Location etc.. There is nothing locking you into what parts of the Parse SDK you use.

4) You can always export your data.