Apparently, The Carpenters actually recorded the whole song, including the intro. It's on this album:

"Christmas Portrait" is the first Christmas album recorded by The Carpenters, originally released on October 13, 1978 (A&M 4726).

WHITE CHRISTMAS
(Irving Berlin, ©1942)

Intro:

The sun is shining, the grass is green,
The orange and palm trees sway.
I've never seen such a day in Beverly Hills, L.A.,
But it's December the 24th
And I am longing to be up north.

Verse:
I'm dreaming of a white Christmas
Just like the ones I used to know,
Where the treetops glisten, and children listen
To hear sleigh bells in the snow.
I'm dreaming of a white Christmas
With every Christmas card I write,
"May your days be merry and bright
And may all your Christmases be white."

"White Christmas" - The Carpenters

I actually hear this recording sometimes on radio at this time of year.