I had a trip planned to Hawaii for DS's wedding when I got my diagnosis. I planned with my oncologist on my chemo treatments and lukine shots so that I was still able to go to Maui. Yes, I was bald and I had a few days that were tougher to get through because I was tired, but I did not cancel my trip. I wore a ball cap to cover the noggin and had to skip a few daily injections (with approval of doctor) but other than that, the quality of time on this vacation was no different than any other I'd taken.
I wouldn't cancel anything at this point. You might find, as I did, that the break from the daily grind of treatment ended up providing me with some R&R. Food will not taste right but the vacation may be very good for you.
Good luck. I'm off to WDW in Jan to celebrate one year from my surgery and the fact that 2007--the longest year I've ever had--is finally over!!!!