To cut a long story short, if you can discover surrounding wifi network but not yours, then here is a quick fix to wifi problem I discovered myself:
1. Go to "About This Mac";
2. Go to "More info...";
3. Go to "System Report";
4. Go to "Network" -> "Wi-Fi";
5. Under "Interfaces:", Check "Supported Channels:";
If your wireless router is set to some channels that your wifi card does not support, then just change your wireless router to the channel your wifi card supports!
For example, my friend's Netgear 834G was set to "Auto" channel and it was on channel 12, but my Late 2009 iMac does not support channel 12, so my iMac could not even find the wifi SSID. After change channel to 11, then wifi works again!
I couldn't tell if Apple has dropped channel "12" and "13" in Lion or not, since my iMac's wifi worked fine in Snow Leopard with my friend's Netgear 834G on auto channel. So this is just a quick fix for those of you experiencing this type of issue.
