(Warning: long post follows)
Actually, now that I look through the different rulebooks it seems not being allowed to "disclose the content of a hand" is the norm rather than the exception.
Note though that this isn't necessarily the same as telling the truth about your hand. Whether telling the truth about your hand without physically turning the cards over counts as disclosing it is, I guess, up to the individual floor person or tournament director but if they wanted to be nitty about it the case could certainly be made.
Anyhow, here's the relevant sections from three different sources (emphasis / comments added by me, obv).
Robert's Rules section 1, Proper Behaviour, Poker Etiquitte:
The following actions are improper, and grounds for warning, suspending, or barring a violator:
(Bunch of different things, including...) Revealing the contents of a live hand in a multihanded pot before the betting is complete.
Robert's Rules section 15, Tournaments: (http://www.homepokertourney.com/roberts-rules-of-poker.htm#SECTION_15_-_TOURNAMENTS)
22. Showing cards from a live hand during the action injures the rights of other players still competing in an event, who wish to see contestants eliminated. A player in a multihanded pot may not show any cards during a deal. Heads-up, a player may not show any cards unless the event has only two remaining players, or is winner-take-all. If a player deliberately shows a card, the player may be penalized (but his hand will not be ruled dead). Verbally stating one’s hand during the play may be penalized.
Poker Tournament Director's Association Rules: (http://appservicesonline.com/PokerTDABlog/wp-content/uploads/2009/10/Poker_TDARules_2009-Version2_Final_longform_pdf.pdf)
41. No Disclosure
Players are obligated to protect the other players in the tournament at all times. Therefore, players, whether in the hand or not, may not:
1. Disclose contents of live or folded hands,
2. Advise or criticize play at any time,
3. Read a hand that hasn't been tabled.
The one-player-to-a-hand rule will be enforced.
2011 WSOP Rules: (http://www.wsop.com/2011/2011-WSOP-Rules.pdf)
Rule 95: Players are obligated to protect the other players in the tournament at all times. Therefore, whether in a hand or not, players may not a.) Disclose contents of live or folded hands, b.) Advise or criticize play before the action is completed, or c.) Read a hand that hasn’t been tabled. While in a hand, players may not a) discuss hands or strategy with any spectator, or b) seek or receive consultation from an outside source. The one-player-to-a-hand rule will be enforced. Players who violate this rule are subject to penalty in accordance with Rules 38, 94, and 98.
And just for laughs, here's the rule from my local casino (which hosts the Aussie Millions and a few other major international tournaments)
Crown Casino Poker Tournament Terms and Conditions: (http://www.crownpoker.com.au/Assets/ContentFiles/898/Crown%20Poker%20Tournament%20Terms%20and%20Conditions.pdf)
7.1 The Tournament Director may penalize any entrant who fails to comply with these terms and conditions or whose behaviour is assessed as being detrimental to the integrity or the proper conduct of the tournament, including but not limited to the following examples:
(Some other stuff, then) 7.1.3 An entrant showing his/her card(s) or verbally advising the content of his/her hand to another entrant(s) when all action for that round is not complete.
How often these rules are actually enforced and how harsh the penalties are is another question entirely, of course. "May be subject to penalty" is vague enough in itself.
Remember too that, as mentioned above, these rules don't make specific reference to "telling the truth" about your hand - it's instead left to the discretion of the tournament director or floor person to decide whether telling the truth constitutes disclosure (though the Crown rule and Robert's Rules tournament sections specifically mention verbal statements).
These are the current versions of the rules, BTW, and I'm lead to believe that previous versions did
make specific reference to not being allowed to tell the truth. Public rants by Mike Caro and Steve Zolotow, among others, apparently resulted in them being changed to something like the versions you see above.