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:
In the event ephsmith's doesn't work I found an additional issue with my own setup.
My macbook wouldn't connect after my AP decided to reset it's settings. During the configuration, I'd set the SSID before enabling WPA. This caused the macbook to remember the unsecured AP only and wouldn't automatically join the WPA protected instance. By removing the unsecured access point from the list from "Preferred Networks" (and ensuring "Remember networks this computer has joined" (as mentioned above) fixed the problem.