XBox Live Rewards is running a promotion
for the month of April in which XBL Rewards Members (sign-up is free) can earn free stuff.
Play 20 hours of any combination of arcade games, receive a free avatar item
Purchase any four arcade games for 400 Microsoft Points or more from Xbox LIVE, receive a one month Xbox LIVE Gold membership
Spend 3200 Microsoft Points on arcade games, receive 800 Microsoft Points in return
Although this information is from Majornelson's blog, the last item doesn't quite match the requirements on the XBL Rewards page. There, it says you have to buy 4 XBLA games that cost 800 points or more to receive the free 800 points.