What can cause 90 second delays joining a wifi network? I have been using my Mac Book Pro (15" Lion, latest version) for coming on a year.
I used to open the cover, it would search for my wifi, and in 15 seconds I could start web surfing.

Suddenly, in the few weeks though, it is doing something strange...

It still finds the wifi connection in a few seconds (the Wifi icon shows all dark), but now it takes an extra 90 seconds plus until I can actually load anything in a browser.

I noticed that Skype connects after about 90 seconds, then a few seconds later Safari allows me to start loading web pages, and then a few seconds later Chrome will let me start loading web pages.

Very nerve wracking! Any suggestions for diagnosing this or speeding this back up again?
You can also boot up to the start up disc that come with your mac if it is 10.6 of newer. There you can connect your wifi when the disc has loaded up to check make sure its not a software issue.

You can also try resetting the P-Ram on startup, by holding P + R + cmd + alt and letting its chime 3 or 4 times. That may help.

Also check with other machines to see how long it takes to receive an ip address. This may be an issue with the router.