In my experience with a similar issue (albeit without using WEP at the time) the problem was caused due to some preferences error or other. I believe I had to delete a batch of cache files and prefs files to get things working again.

The problem manifested itself after having gone over to a friend's house and properly connecting to their network. For some reason after returning home my machine would no longer automatically connect to my network. Every time I booted the machine I'd manualy have to pick my wireless connection from the wireless menu.

Another symptom of this issue was that my Airport pref pane was missing an option or two. It wasn't presented the same as on another of my machines.

Currently (with everything working) I have my Airport prefs set to "By Default Join Preferred Networks" - in the network list I have numerous networked listed, with and without encryption. My home network using WPA2 is listed first, above everything else. In the "Options" window I have only "Ask before joining an open network" selected and the "Automatically add new networks to the preferred networks list" checked.

You might consider manually deleting all preferred networks from your list while disconnected and then reconnect to see if re-adding (manually or automatically) fixes the issue.

So do this before exploring manual file deletion:

0. Disconnect from the wireless network (and/or turn off Airport from the menu extra at the top of your screen)
1. Open System prefs
2. Select Network
3. Select "Airtport" in the Show pop-up menu
4. Make sure "Airport" is the selected tab
5. Select "Preferred Networks" from the "By default join" pop-up menu
6. Select the first listed network and press the "-" button to delete it.
7. Repeat #6 for all listed networks
8. Press the Options button
9. Select "Ask before joining open network" in "if no..." pop-up menu at top.
10. Check the "Automatically add..." item
11. Uncheck all other items
12. Press OK.
13. Press "Apply Now"
14. Close System Preferences
15. Turn on Airport
16. Select the SSID for your desired network from the Airport menu at the top of the screen (SSID broadcasting should be on in the access point/router - if not, select the "OTHER" item from the menu
17. If you selected the network by name it should identify the type of protection being used and prompt you for the security key/password - otherwise you will type the SSID into the "Network Name" box of a connection window and select the encryption type from the "Wireless Security" pop-up menu. Then select "show password" and type your password into the box provided.
18. When you press OK it should connect.
19. Put the computer to sleep.
20. Wake the computer and it should reconnect to the network.
21. Restart the computer and as in #20 it should reconnect after logging in.