There are two places that I go to when wanting to determine what Canon
cameras are currently hot in the digital camera market: DPReview.com
and PBase.com.

At DPReview.com, a list of the 10 hottest cameras being clicked on at
the site (click popularity indicates the level of curiosity for a
particular camera model) is an approximation of the relative interest
the buying public has with respect to a particular camera model.
Another thing to note is that typically, only the newest range of
cameras appear in this list.

As I write this, Canon currently occupies 9 out of the 10 spots (the
only non-Canon occupying a place there is the Nikon D80):

Canon PowerShot SD800 IS (announced: Sep-2006, 7.1 Megapixels,
28-105mm zoom range - the 28mm is especially useful for wideangle
shots, Image Stabilization - also known as Shake Reduction)
Canon EOS 400D (announced: Aug-2006, 10.1 MP Digital SLR camera,
second smallest and lightest digital SLR body on the market next to
the Nikon D40)
Canon PowerShot G7 (announced: Sep-2006, advanced "prosumer" 10 MP
compact digital camera, 35-200mm zoom range with Image Stabilization)
- read more on how the G7 compares with the PowerShot A640)
Canon PowerShot SD900 (announced: Sep-2006, 10 MP, titanium body, also
known as Canon Digital IXUS 900 Ti in Europe and IXY Digital 1000 in
Asia, 37-111mm zoom range)
Canon PowerShot SD600 (announced: Feb-2006, 6 MP, 35–105mm zoom range)
Canon PowerShot S3 IS (announced: Feb-2006, 6 MP, 36-432mm zoom range
with Image Stabilization)
Canon PowerShot A710 IS (announced: Aug-2006, 7.1 MP, 35-210mm with
Image Stabilization) - read more on the Canon A710 IS compared with
Canon A640)
Canon EOS 30D (announced: Feb-2006, successor to the EOS 20D, 8.2 MP)
Canon PowerShot A640 (announced: Aug-2006, 10 MP, 35-140mm zoom range)
The list at DPReview gets updated almost daily, and it's not uncommon
that a newly-announced digital camera shoots up to the Number 1 spot
in a matter of days after its announcement. Over the past few years,
I've noticed that Canon always holds the dominant position in terms of
volume of clicks.

Another site that serves as an approximate barometer of camera
popularity is the "Hot Cameras" section of PBase.com - see it here.
This too gets updated daily, however the statistics here show a
different picture - Digital SLR cameras rule here, to put it mildly.
These stats are very different to those at DPReview.com, instead of
showing how many people are interested in a specific model, it
displays the number of times photos taken with a particular digital
camera have been viewed. The conclusion one can draw from this is the
amount of use a particular camera model gets after purchase. Digital
SLR cameras typically see extremely heavy use due to wedding and event
photography by professionals and enthusiasts, who then post the
pictures up at PBase for review by clients. These pictures, very often
organized into albums for specific events or wedding shoots, also act
as a showcase for their services.

The current Canon cameras which are popular at Pbase.com include:

Canon EOS 5D
Canon EOS Digital Rebel XTi
Canon EOS 30D
Canon PowerShot G7
Canon PowerShot S3 IS
Canon EOS 1Ds Mark II