Ensure that WiFi is switched on in the BIOS.
I had this problem with the Asus 901.

Finally got the chance to have a look at this (working stupid number
of hours at the moment) - I cannot find any reference to WiFi in the
[somewhat limited] BIOS.

It's interesting that the WiFi stopped working when I installed UNR
9.04 (with a network cable connected, rather than the WiFi)

Interetsingly, the battery has also given up the ghost, ironically
around the same kind of time (although, I understand that this is
something that's not uncommon with these Acer Aspire One batteries ...

pc are you on that you think wireless would be in the bios?

The PC in question is an Acer Aspire One A150 ...

I was looking in the BIOS on the suggestion of another list member,
that's all ...

I also have the Acer Aspire One duel booted with UNR and Win XP. If you look
at the front right there is a switch try playing with that It has something
to do with turning the wireless on and off. I have had no problem with the
wireless on UNR but a virus wiped out all of my network connections in
Windows, of course now where with enough space to dump all my MP3s, MP4s,
pictures and tons of Genealogy data so I can reload Windows XP. My problem
on UNR is Flash pages and streaming video, Youtube, Hulu, and others. Let me
know if that switch thing works.

I'm having the same problem, Once I had a wireless connection, Now I don't,
Don't know when it was lost, Could have been when I updated to 9.04, WinXP
still works so the hardware still good.

This may be a dumb question but how do you get to the BIOS settings on an
ACER Aspire One A150?

With 9.04 on my daughter's Acer Aspire 5100 Wifi does not work. With 9.10 on
the same laptop WiFi does work.

Each upgrade comes with new applications. Network manager is replaced. The
kernel is replaced. In short you need to reconfigure it.

See: https://help.ubuntu.com/9.10/internet/C/connecting-wireless.html

and if you have problems beyond setting it up:
https://help.ubuntu.com/9.10/internet/C/troubleshooting-wireless.html

On boot, hit F2 - that should take you into the BIOS (or SETUP, as
it's referred to, iirc).

F12 also brings up the boot menu, if you haven't found that one yet.

Mind you, you need to be quick, as it very quickly passes this, and
goes into GRUB ...

Yes, I'm aware of the switch on the font - this is what seems to have
stopped working - for some reason, I've always found wireless somewhat
unreliable at home (something has to be interfering with it - have a
pig of a time with mobile phone signal reliably as well), so I tend to
leave this switched off, and simply use a UTP connection.

However, when I tested things after the UNR install, I found that the
wireless wouldn't even switch on ...

Ok, a bit of a development - brought the lappy into work at the
weekend, so that I could play with it here - eventually managed to
connect to the WiFi, but the LED on the front appears to be very
erratic as to when it actually works.

The wireless connection was rock-solid stable - no drop outs or
anything - even managed to upgrade to 9.10 over it with no problems
whatsoever.

Back at home, I tried connecting again - ok, no problem (aside from
the aforementioned LED) but, after a while, the wireless connection
dropped out, and wouldn't reconnect - had to resort to a UTP cable
again.

The bizarre thing as that I don't happear to have any problems using
the WiFi at work, only at home, yet other people don't have any
problems using the home one ...

This is doing my noodle in ...

You might try installing and running wavemon to check how much noise you have in
your home environment.