Each time I need to go with my 3G when we leave our WiFi at home I have such a problem when I come home and wanna use our WiFi again. The iPad sees my WiFi but will not connect up. It keeps saying it cannot. When I check it out in the settings I find the network there but no router of DNS numbers!
So, I have to reboot my router and forget my network on the ipad, and restart them both till the iPad finnaly sees the network and hooks up fully!
Why? I have updated firmware on the router, iPad is updated all 'round, Wifi security PW is correct, etc..
Should this not work seamlessly each time I need to switch from one to the other ?
Thanks for any replies and help!
Joe

I have a 3g/wi-fi device as well, and while I haven't tried the 3g connection ( due to $) i can see where something like this might occur. I know from working with iPhones with both wi-fi and 3g enabled, that the device does not do a very good job with switching between different communications channels...I'll take alook on the forums and see what I can find.

one other question; do you turn off 3g when you come back home to the wi-fi network?

Yes I do turn off the 3G when I come home. I do this most often but sometimes leave it on because I thought that the iPad would defer to WiFi if it's there and not be use up my 3G..not certain though on that one. So, the answer to the question is YES.
TY for your interest.
Joe

Yes, it will switch automatically. I leave my 3G on all the time and my iPad switches wherever I am.
An example - It uses WiFi at home, switches to 3G as I drive to the airport. In the airport it uses the SkyClub WiFi, then switches to GoGo Inflight on the plane. When I get to my home office in AZ, it uses the in office WiFi. Same thing on the trip back home until it finally hooks back in to my home AP.

A couple of things I have noticed -

-It can sometimes take a little bit to hook into the AP after being out for a while. I have 4 APs scattered around my house, one an 802.11N. My iPad usually uses the N AP.
-It also doesn't always see the N AP immediately so it grabs one of the other ones. I have to go in and manually choose the N AP.
-It can sometimes take a little bit to get all of the IP info from the router.
-Sometimes you need to turn off WiFi in Settings, then turn it back on and WiFi works better.
What Firmware version are you on. Someone else would have to speak to that but I remember that some units had flaky WiFi on the original firmware.
What wireless AP/router do you use?
Keep searching and working on it, you shouldn't have these kind of issues.

Check out this thread on the Apple Support forums...it appears that these issues are "known"...and there doesn't seem to be a fix, at least not one that has been posted....

Apple - Support - Discussions - iPad 3g won't reconnect to WiFi Router ...

iPads can be quite finicky about Wi-Fi in general though, and I had more issues with mine and getting it to connect than any other Apple device I've ever owned (my model is the straight 16GB Wi-Fi).

At first my iPad resolutely refused to connect to my Wi-Fi network, despite my MacBook, Touch and every other wireless device having no issues. Like in your case, it could see the network, but after entering the password would say it cannot connect (this is with WPA). I went through all sorts of hair-tearing, router reboots, soft iPad reboots etc.

In the end I found the solution was to go into the router settings and simply change the password - same type of encryption etc., just change the letters and numerals, and after that no further issues.

Bizarrely enough, it seems my iPad simply didn't like my password! Worth a try to see if it solves your issues - if not at least another poss. cause is ruled out.

if its not working, put it in airplane mode for a few seconds and then turn airplan mode off and wifi back on.

hmm, I think you can turn to read this article "Tips to Turn iPad Wi-Fi Into 3G"-->Tips to learn how to get your iPad WiFi connected to 3G network, might can do some help for you.