The rules were changed in 2010 to allow texting/tweeting.

Here's a link to this year's official WSOP rules: http://www.wsop.com/2012/2012-WSOP-Rules.pdf (see section 61 on Communication)

Here's the relevant part: "Participants not involved in a hand (cards in muck) shall be permitted to text/email at the table, but shall not be permitted to text/email any other Participant at the table."

Arguably, a public tweet could be interpreted as a text or email to another participant at the table, but the rules are not enforced that way and tweeting is a generally accepted practice by pros and amateurs alike.