Update with bad news and good news -
As with some other companies, the notice that my forks were "shipped" merely meant that the order had been processed and the shipping label had been sent to the warehouse. I got a call this afternoon from a guy at the warehouse. It turns out that the 42" forks are backordered, and won't be in for about 6 weeks. The good news is that he offered to upgrade me to the the 48" forks for a very attractive price.
I believe my JD 4600 with 460 loader will handle the 48" forks just fine. I would have ordered that size anyway, but I thought the $125 price difference listed in the web site was a bit steep.
The warehouse guy told me that my 48" forks, on a JDQA frame, should be physically shipped tomorrow. I'm a happy camper!