The following links are to a couple of photos taken this afternoon, March 2 at 3:44PM, from the southwest side of Oklahoma City looking NNE.
(links removed)
Is this really mammatus? I was surprised to see this formation since the temperature at the time was about 52 degrees.

Update. On 3-8-04 the photos were removed from the website. Contact me via email if you want to see them.
Yes those are mammatus clouds. Mammatus don't just form under anvils, they can form under just about any mid or high level cloud deck.
We see quite a bit of non-thunderstorm mammatus here in West Virginia during the winter months, in fact there were some faint displays of it this morning after the frontal passage.

Back in January there was a very spectacular display at sunrise that I didn't get photos of because I was already 1 hour into a sunrise timelapse.

None match the beauty of a storm anvil display, though.
Can we expand on this thread a bit? There seems to be some confusion over mammatus clouds and what they represent here in Canada. Can someone give a firm answer as to what they are and what they do?

Personally I believe they are hail producers.

We get a lot of cold-core mammatus out here in Oregon during the winter/spring season. I would really love to see some nice thunderstorm based stuff tho.
Mammatus in and of themselves do not indicate a hail producing storm. Mammatus are formed by sinking air - - - for them to form, the sinking air has to be cooler than the air around it and have a higher liquid content. When seen under an anvil of a thunderstorm, the mammatus usually indicate a strong updraft, so there will likely be a hail core on the storm simply because the updraft is strong enough to support water droplets circulating back and forth inside the storm tower and growing as hail.

As was noted by others, mammatus-type formations can also be observed under cirrocumulus, altostratus, altocumulus and stratocumulus clouds.

Last week in Kansas City we had the passage of a super interesting formation. It was a high shelf, with waves that appeared to be like mammatus ... there was virtually NO downdraft associated with this shelf, though from all appearances it looked totally severe! ... Everyone who saw it remarked about it ... it followed me south from St. Joseph all the way to work ... when it reached KC it developed these super cool wave formations that literally made it look like the surface of the ocean. Was quite beautiful and quite interesting. (and yes - of course I got pics!)
Mammatus clouds gets its name, because of the puch like or baglike sacs that resemble cows udders. MOOOOOOO goes the cow. LOL Mammatus form in sinking air, when precip is carried aloft, is spread out at the top of the anvil, the precip particles composed of water and ice and since air is saturated and is heavier, it sinks. Sinking air warms, and the warming is used in evaporating the precip particles, if more energy is needed, the sinking air will be cooler than the surrounding air, and with relative humidities of 100 percent, you will see mammauts clouds. Mixing, evaporational cooling and virga seem to play a role in mammatus.

Mammatus often form on the underside of cumulonimbus clouds, especially underneath the anvil and seen more often with storms that are severe, but you can see mamatus with non-severe storms too. Mammatus also sometimes they can form underneath altocumulus, altostratus, cirrus, cirrocumulus and stratocumulus clouds. Mammatus usually are an indication the storms are weakening and threat for severe weather has passed, however they are exceptions to the rule, you can sometimes have mammatus on the backside of a thunderstorm, where you might have the storm dropping hail and/or storng winds. Mammatus are not a sign that a tornado is about to form. Mammatus clouds do not produce severe weather.

Source: Notes I have collected over the years.