One persistent problem that some users have noted in OS X Lion is where Wi-Fi connections may not autoconnect when waking from sleep or when performing other tasks where the Wi-Fi connection is cut and needs to be re-established. Instead of locating a previously used network, the Wi-Fi connection will remain gray and require you to manually select a Wi-Fi network in order to connect.
If this problem happens to your system, you can try a few things to correct the problem:
An easy step is to try creating a new network configuration in the Network system preferences. Do this by selecting "New Location" from the "Location" menu, and then clicking the plus button to add the network ports that you use (e.g., Wi-Fi, Ethernet, etc.). With your ports added, configure them accordingly, and see if the problem with re-establishing a connection persists.
Remove Wi-Fi keychain entries
Part of the process of establishing a connection with a Wi-Fi network is to supply the appropriate password for the connection. In OS X these passwords are stored in the user's keychain, and if there is a problem with them or with accessing them, then the Wi-Fi connection may not be properly established. Therefore, open Keychain Access and remove any keychain entries that are associated with your Wi-Fi networks, and try connecting again to save your passwords for these networks.
Remove AirPort configuration property list
Many times problems with system services can happen after an upgrade if some corruption has occurred in the configuration files associated with those connections. In the case of Wi-Fi network connections, the options for auto log-in and other details about a specific network are saved in a configuration property list at the following location:
Try disabling AirPort in the menu bar, go to that folder and remove the "com.apple.airport.preferences.plist" file, and then re-enable AirPort. Doing this will create a new configuration file that will not have any residual settings or options in it from your Snow Leopard installation, and therefore may prevent the autoconnect problem from occurring.
This issue is likely not related to Wi-Fi connections randomly dropping for some users, but the solutions here may help those users re-establish their connections if and when they do drop out.