After a bit of encouragement from Gordon I went after Comet Holmes last night. Sky & Telescope have a nice web page up here
at the moment describing how to find it but if you live much to the south of the equator I'm afraid you will be out of luck. NASA's Jet Propulsion Laboratory also has an interesting web page here
for the more numerically inclined, and it includes a pretty neat Java applet showing where in the solar system the comet is (it's well outside the orbit of Mars and about 1.6 times further from us than the Sun is).
The following image is a 33% crop (clickable if you want to download the 100% crop for a bit of pixel peeping) of a 180 second exposure with my Canon 40D set to ISO 1250 and Daylight white balance and with my EF 70-200mm f/4L IS USM lens set to f/4 and 200mm. Apart from the crop the only post-processing was a very small tweak to the levels with Photoshop to darken the sky slightly but essentially what you see is what came out of the camera. No noise reduction or fancy image stacking and dark frames for this one!
To give you an idea of the scale, the comet's apparent size is a bit smaller than the Moon's ½°. Of course it helps hugely that the camera was piggybacked on top of my telescope to take advantage of the mounting's ability to compensate for the Earth's rotation. One note about the lens: it was pretty much vertical during the exposure and I'm delighted to note that there is absolutely no sign of "zoom creep".
As a bit of fun, I took the image above and simulated what I think you would see if you fixed a camera to a tripod. I assumed you could set ISO 3200 and with an exposure of 35 seconds at 200mm and f/4 you should, after a bit of processing, see something like this 25% crop:
I've probably got the trail (motion blur) direction wrong but you get the drift.
Why not have a go and see if you can do better? For Nikon D80 owners you might want to review this post
which describes how to defeat the long exposure noise reduction issue.
In any event, if it's a clear night and you live in the Northern Hemisphere why not grab a pair of binoculars and try some Mark 1 eyeballing. When you've ticked off the comet there's lots else to see at this time of the year, particularly if you wait until the Moon has set.