Very sporadic Wi-Fi Issue with Lion (Connection Timeout) I have tried many a thing to solve this matter. Long and short of it is I am getting the standard "connection timeout error" but only on some networks (which are successfully connected to by other Apple devices). 

What I have done (after extensive reading) and things worthy of note:


More power cycling than I can count. Both of Wi-Fi and computer.
Creation of new places with various changes or not changes in Network preferences. Manual resetting of DHCP Lease or adjusting of MTU's as per other suggested fixes.
Deletion of SSID related files in the Keychain.
Deletion of network configurations as per other recommendations.
Reset NVRAM.
Fresh install(s) of Snow leopard (x2) and Lion(x2).
Checked religiously for updates on software.
Used Network Diagnostics, Assist Me, and Join other network (Both with attempts with accurate password and inaccurate. The dilemma seems to be that it won't successfully make contact to my home Wi-Fi, even to tell me if the password is correct).
Wi-Fi works almost everywhere but my home (regardless of how close my computer is to the station or if all other nearby devices that can cause interference are off).
Did not have this issue with Snow Leopard 10.6.0, but became an issue upon an update (which is required for any form of computer use that involves the internet as I discovered as every file out there required the highest end of the operating system to be used, such as Flash or Java).


My computer is a late 2007 MacBook that cannot run an OS higher than Lion.
OK, lets try to Manually connect to your WiFi.

Use following in your Terminal app (located in the utility folder).

Turn On (skip this if it is on already)

networksetup -setairportpower en0 on


Find (skip this if you know the WiFi name and the passowrd)

/System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport scan


Connect

networksetup -setairportnetwork en0 WIFI_SSID_I_WANT_TO_JOIN WIFI_PASSWORD


Just replace the "WIFI_SSID_I_WANT_TO_JOIN" with your WiFi name