In my experience the difference between an Architect and an Engineer is usually quite significant. As far as job duties go, the engineer will be responsible for calculating all the necessary loads, requirements, etc. for their particular discipline while the Architect will be responsible for creating the general "feel", "asthetics", and process flow of the building. Also, the Architect is responsible for the overall management (yes, I know the engineers are cringing) of the project including design budget. I have also seen WAY too many cases where the Architect is responsible for maintaining the budget AND is the reason why the budget gets overrun ALL IN ONE. I think the other folks who have responded are also correct here. Creative Architects and Engineers may be able to cross crafts if owners, building inspectors, and codes allow. Some projects require the AIA certification.
On a second note, I have also seen the difference in Engineers and Architects in the way that they design. An Architect tends to thinks in a circular direction (picture a screw thread) where their design revolves and evolves in an ever decreasing spiral until they narrow it down to the final design. An Engineer is more finite. They calculate a load, crunch some numbers, and viola, there is your answer. An Engineer thinks in a straight line. Point A to Point B. I also joking making the statement (sorry for any offense in advance) that the Engineer can prove their design while the Architect can't. You can prove how much load a beam can carry mathmatically while you can't prove that a room painted green is better than a room painted blue.
Just my thoughts,
Paul