Use this letter if you're in an industry that resents employees who leave for a philosophically different path or industry.
Dear Mr./Ms. Bossname,
The rumors are true! I interviewed with (Company) and I will be going to the "Dark Side." Please accept my two weeks' notice.
I know it will be hard to transition to (sales/public relations/etc.) after working in (HR/journalism/etc.), but I've steeled myself and am up for the challenge. Many have gone before, lured with promises of higher pay and delicious cookies.
Please don't play the Imperial March at my going-away party. I'm still the good person you know and tolerate.