Being a Nikon user already I am surprised why you are not going for a Nikon Digital Camera? Have you considered any Nikon model? Besides Canon S45 which other cameras were you considering? I would love to hear you out.
As for the Canon model you mentioned I am not sure why you picked this particular one. I suggest you weigh the pros and cons, being a photographer it should not be a problem for you to decide.
1) Are you into macro? If so, this is a poor choice, if not, its ok.
2) Vignetting at wide angle and maximum aperture could be a problem if you are into landscapes and such, if not, you are ok?
3) Good natural light white balance, strong and balanced colours make this a great camera for outdoors, if you mainly intend to shoot outdoors this is a good choice.
4) Good manual control so it may prove very useful if you want to be in command than just be a snap shooter (which you are not)
Let my know your opinion to what I have suggested and we can take it from there.