My iPhone 4 will not connect to my wireless router EVEN THOUGH I'm using the correct password. It tells me it is the wrong password. All other devices (netbooks, laptops) connect with no problem.

reset it

Solution - reset the network settings Settings -> General -> Reset -> Reset Network Settings
This will remove previous settings and bugs related to connecting to the wireless, try reconnecting, and see if this solves the issue for you.
If it does not, connect your iPhone to iTunes, backup your iphone, and perform a full 'restore'.
This will solve the problem, unless there is a hardware issue with the wireless module inside the iphone.

I wonder if your router is set to perform MAC address filtering. If so, you have to either find the MAC address of the iPhone's wireless card and add it to your router's appropriate list, or set your router to not filter by MAC addresses.
Just my 2 cents... Carlos

If you have a 3G phone in the house, It operates at the same frequency and is a nono.

If not I would suggest you completley rest modem and re-enter U/N, P/W etc and set up secure network also. If still a problem, maybe faulty modem.

Use a different type of security like WPA or WPA2

Make sure that the password that you have is correct if not then you will need to check the password of the wireless network.
Click Here for more info about wireless network.

That's firmware problem on your router. Try to download the latest firmware for you router from www.linksys.com/download then load the latest firmware. After successfully updated the firmware, you need to hard reset the router by pressing the reset button for 20 seconds then unplug the power for 10 seconds and plug it back in. You will need to reconfigure the wireless settings.

WEP is a standard encryption standard that has a 4 keys based on a passphrase. In addition to the correct name and the correct key, make sure youre picking the correct numerical key. (Example: Youre connecting to network MYHOME and the passphrase generated 4 keys and the router is using key number 3. You must use the third key.) Look up the settings on the router and retrieve the encryption key, and whether its the first, second, third, or fourth key. Youll need a computer thats directly connected to the router.

You must be sure to what router you are trying to connect. I understand you have a ZyXel and a linksys. You must have 2 wireless networks. Your wireless network must be secured, so you just have to use the correct password. I need more details. Try this http://technicalsupport.blog.friendster.com

Yep, I had the same problem with my iPhone as well (32GB 3GS).
Had the signal NO problem for about a week then POOF it was gone. WTF?
Prowled the boards looking for a solution.
Tried resetting the phone, resetting the Wi-Fi, switching airplane mode on and off, forgetting the network, etc.

HERE'S YOUR SOLUTION :
Go unplug your wireless router's power cord.
Wait 30 seconds.
Plug it back in.

Your iPhone will reconnect, and you'll be back in business.

Jack