We love the flying pigs as well.
Here are our efforts (although taken with a Nikon 995, as we weren't going to take our precious into all that dust and bustle).
You'll see that, where the pig becomes more airborne (ie picks up speed), we had difficulty with focus as well - with the 995, you really have no choice but to try and pre-focus on the anticipated spot, as it's much slower than the D70. For example, in attempting to take photos of the pig racing, we have one that's far too early, and three of empty track after they'd passed
BTW, I know these are underexposed, but I didn't think they were really worth the effort to fix - just posted them to show that you weren't the only one to suffer the "flying pig out of focus" problem ... and we were stone cold sober, officer, I swear.
Did you also notice, that in the farmyard, there was a photographer taking photos of children with baby animals (set up with bales of hay etc). They were charging quite an amount for each photo - looked about 10 x 8. Anyway, they were using a laptop, a D70 and the kit lens. Even quite late in the day, they weren't short of takers, either.