I have anxiously awaited this “Always On” feature, previously discussed in the issue forums, and was finally announced on Dec 2 with release of App Engine version 1.4! I was getting seriously screwed by JAXB in the GoogleCheckout java library, which I blogged about previously.
So I am now paying $9 a month to keep some instances “Always On”. My control panel has an extra spot on the Instances page now:
I am still awaiting Google App Engine for Business, so that I can have a SLA for my apps. Surely this “Always On” feature will be standard since GAE for Biz will be a paid service. I certainly think any app using JAXB has the potential to require an “Always On” approach to make it function on AppEngine without timing out.