I know there was a fanpost about the Sun article already but now Josh Gross is reporting this too so it looks like there is some truth behind this.

A championship battle of undefeated light heavyweights is slated to headline UFC 98, SI.com has confirmed. Rashad Evans (13-0-1) will defend his UFC belt May 23 against Lyoto Machida (14-0).

http://sportsillustrated.cnn.com/2009/mma/03/11/evans.machida/index.html?eref=T1

fanpost about Sun article: http://www.bloodyelbow.com/2009/3/10/789187/rampage-out-of-fight-with

Edit: Sam Caplan is also reporting it from his sources now too, looks like this is going to happen: http://fiveouncesofpain.com/2009/03/11/sources-rampage-to-undergo-surgery-evans-vs-machida-expected-to-headline-ufc-98/