My s400 (practically the same cam with 4.0 MP) takes images at 180 DPI as well. That's what it saves the JPEGs at, no way to change it, you'd just have to make them 300 in Photoshop.

36x27 inch graphics at the resolution you mention works out to 72 DPI, so I guess Illustrator isn't respecting the DPI of the files (I don't think this is correct behavior, but I'd have to double-check that). If you're going to be printing these, 72 DPI is a no-no!