This is not as big of an issue as many seem to think. Perhaps if you drive an exotic car, but for regular cars, that don't stand out, I don't think it's a big deal.
When I imported my Mazda 3 from Canada to US, I drove on Canadian plates for several months while I was waiting for the DMV appointment and then their temp plates. Of course I had all the proper paperwork.
The only incident I had was about one week after I imported the car, an officer pulled up next to me at a red light, rolled down the window and asked how long have I been in the US. I replied one week, and he said to remove the plate covers, as they are illigal in CA. I thanked him and removed the covers that same day.
I never had an issue with my Canadian plates.