Macbook Air does not automatically connect to known networks after wake up
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:


Visit System Preferences -&gt; Network,  select "Wifi" and click the lock (if needed) to authenticate. 
Assure that "Remember networks this computer has joined" is checked
Note the order of the "preferred networks" (see blue box in the figure).  Re-order these appropriately.  If you'd like to start fresh, just clear the list. 
Click "Ok" to accept the changes