This wasn't just sent to Jim Cameron. Lucas bought a full-page ad in Variety. That's why this looks like a scan from a magazine.
This is actually part of a decades-long tradition...
When the original Star Wars beat the box office record held by Jaws, Speilberg took out a full page ad in Variety that showed R2D2 reeling in the eponymous shark (thanks to toefer for posting this one, too).
Later, after ET retook the record, Lucas ran an ad with R2D2 putting a crown on ET's head.
Then in 1997 when the Star Wars Special Edition regained top-grossing spot, Spielberg's congratulatory ad showed a long line of movie characters with R2D2 zipping past to the front of the line.
This ad of ET returning the crown to R2D2 fits in there somehow as well. I hadn't heard of this one before toefer dug it up.
I guess it's time for Jim Cameron to run an ad congratulating himself for beating his own record.
If anyone can find scans of the other ads, I would be most grateful.
Others:
Source:
1998: Lucas sent this to Cameron, when Titanic dethroned Star Wars at the box-office : pics
Avatar has now broken the record set by Titanic. It is now the highest earning movie EVER!