I have a new 2012 Macbook Air (with 10.7.4) which doesn't connect to my own local network automatically after it wakes up. I have to choose my network manually and it works instantly.
Any ideas how to solve this problem?
I had similar issues with earlier versions of 10.7 as well as 10.6. I can't guarantee that my problem is a match to yours, but I'll share what worked.
In both cases my network had somehow been lowered in priority in the list of "Known Networks" and the list had become a bit bloated in one instance. Ever since, I've made it a point to somewhat regularly visit and clean up my list of known networks.
Note: What follows may not resolve your problem, but it's a good step to take. If it doesn't, I suspect that it's related to a more touchy Wifi issue for Macs running Lion. I recall that there have been at least 2 updates to 10.7 that addressed issues with reconnecting to Wifi when waking from sleep. I've seen it, but it's rare for the Macs that I manage.
Here are the steps that worked for me: