I was in somewhat the same situation as you. I was studying Computer Science in my undergrad but by the beginning of my 3rd year I was certain that I didn't want to work as a Software Engineer and instead wanted to move towards optometry. I considered cramming in alot of classes towards the end to finish my prereqs but ended up finishing my degree and then spent the next year finishing the prereqs before applying and being accepted to optometry school.
I don't think the fact that you are a Engineering major instead of a biology/chemistry major when applying to schools will hurt you, but like the previous poster said, you should make sure that it's the field that you want to be in and why. That way, when writing your application essay and going to interviews, you've already thought everything out and have support for what brought you to optometry and why you are interested in it.
As far as what to do about when to apply I don't know how many prereqs you have done now and when you plan to take the OAT, but a good place to start would be to study/take the OAT this summer to see how you do and how you feel about starting the application process. If you do have alot of your prereqs done and good grades in those, you could apply early and hopefully it you were accepted, you could find out the requirements from that school (do they require that you finish your bachelors, or can you just finish the prereqs, etc) and adjust your spring semester accordingly. Most of all, I'd say don't be stressed. If you feel that having everything done for the 2011 application cycle is going to overwhelm you and impact your grades, then wait until the next year. This is a career decision you're making, and taking some extra time won't hurt as long as you get there in the end.
Good Luck!