The Canon 100mm f/2.8 is a great lens for macro photography. It is indeed possible to use it on the 450D, as I do so myself. However it should be noted that due to that camera's 1.6x cropped sensor, you get an "effective focal length" of 160mm. This is due to the fact that the 450D has a narrower field of view than a full-frame camera like the 5D or 1D series.

As far as general macro photography goes, the 100mm is a great lens that offers beautiful bokeh (background blur), very wide maximum aperture, very smooth focusing, and a decent focusing distance. Specifically for insect macro photography, the 100mm will allow you to get some ok shots, but if your wife wishes to get in really close, the Canon MP-E 65mm 1-5x Macro lens might be a better chose. A true "macro" lens can magnify a subject 1:1 on the image sensor (in the case of the 450D, its effectively 1.6 times "larger" than 1:1 because of the cropped sensor). The Canon MPE 65 is capable of magnifying subjects even larger, up to 5:1, on the image sensor (or an effective range of 1.6:1 through 8:1 for the 450D.) The MP-E is a more manual lens, as it does not offer automatic focus, while the 100mm does. At very close distances, however, AF looses its value, and manual focus begins to reign supreme.

In comparisons of Insect macros taken with the 100mm vs. the MPE 65mm, the detail and magnification offered by the 65mm is simply stunning. If extreme close-up is the goal, I recommend the MPE 65mm over the 100mm.

EXAMPLES:

Macro close with the 100mm

Beyond macro with the MPE 65mm