I've noticed that with the last two beta updates, (35 and 36), I was not prompted to update in the app upon launch like I usually am. I had to manually go to hockeyapp.net and download it.
We've noticed that as well and are looking into it. We did notice that a few people did get notified in-app about the update, but there was a large delay between us pushing the release and them being notified.
I also have stopped getting in-app update notifications some time within the past three or four versions. The most recent two updates I've had to install manually by clicking the hockeyapp link in the email.
@grahams, the release notes for 4.5b37 include the following:
[FIXED] Fixed our HockeyApp integration. The next beta should prompt everyone to install it in-app.