How about a pop-up page, like I`ve seen a few times when logging in? They`re usually about new features in the pro-shop or special deals, etc..
When ever there is an up-date, it would instruct you to clear the caches as soon as you log-in. This would also be helpful if someone hasn`t played for a while and wasn`t aware of any up-dates since the last time they played. There`s also the fact that you would have to click on the x-out to go any further, thereby eliminating the "no-one told me!" situation. This would leave only the obstinate few to deal with ;).