I'm going to go a completely different route on this answer. I have had 7 old Dell workhorse laptops ( 600, 610, 620, 6000 all still in use by family and friends) and all running Ubuntu. The first thing I do with all of them is go to Ebay and order the Intel wireless card that was an option (they all had one). I decided it was just easier to never ever have to worry about a driver or upgrade again. I have never paid more than $10 shipped and it seems well worth it. They swap out with a small philips screw driver in about 5 minutes. This answer won't get your running this afternoon so don't expect it to be accepted but as a long term solution I highly recommend it.
I have some moral issues with an OS dictating a hardware purchase, especially when the previous [downgraded] version ran the hardware, there's just no excuse for this, I did however use old fashioned instinct and opened up the Synaptic package mangler [pun intended] and typed in Dell. Lo and behold it's a chip-set problem!
After installing the montage of dell system drivers and especially the chipset update whammo!! The wifi card works like a champ. I didn't know Dell had apt-get goodies in our repos but then again something told me to look.
Dell Latitude D620, wn3945ABG wifi card.
Pop out the keyboard and look under it [upper right] if you're in doubt of yours.
Now if your wifi is a different flavor , Broadcom, Lynksys etc... This still might help, Chipset is important for integer and DMA addressing of hardware.
