Airport Express works fine with non-Apple products???

Question

I've owned my Airport Express for 2 years now and never a problem until this morning.  Now, the airport express is powered on, green light and all, connected to cable modem, but my Early 2008 Macbook Pro and iPhone 4 will not connect to it.  Funny though, because it shows up in my list of networks on both devices.  When I try to select it from the list (of course it used to connect automatically, now it doesn't connect at all), neither the MBP nor the iPhone can establish a connection with it.  Sometimes the iPhone asks for a password, even though the password hasn't changed and it is stored in the device.  It should also be noted that both the iPhone 4 and MBP can still see and connect to my neighbors' wireless routers within range.
Now for the really weird part: the Airport Express still connects to and performs at 100% with other devices, including 1) my Dell running Windows Vista, 2) my XBox 360, 3) HTC Thunderbolt phone, and 4) a Sony Blu Ray player with wireless internet capabilities.  I have power cycled all of the equipment at least 5 times, and all of the non-Apple devices continue to connect to the Airport Express just fine, while the iPhone 4 and MBP cannot connect.  I have even installed Airport Utility on the Dell and did a full factory reset of the Airport Express, but that didn't fix the problem either. 
What is going on here?

Answer 1

At first we could plug the Dell in directly but that was before setting up the AX. Now that the AX works the Dell cannnot connect even wired. (It says that it's connected but it cannot find server when you try to open a webpage). Could this be a problem that the moden cannot go between Mac & windows?
Most modems tend to "remember" the last device connected to it, so it's important to do a complete power-down/power-up when switching devices. It's possible that your modem did this when you attempted to connect the PC directly. My purpose of having you connect the PC directly was to verify that it can access the Internet with a wired connection and that it's wireless setting needs to be troubleshot further.
The power recycle process should be something like the following:
- Power-off the modem & AX; If possible, leave the modem off overnight.
- Power-on the modem; Wait at least 30 minutes.
- Power-on the AX; Wait at least 5 minutes.