Your "Reward Credits" expire 6 months after your last activity. Reward Credits are your comps and they are worth 1¢ per point (100 rewards credits = $1) The example below is pasted from my last Total Rewards eStatement.

Last Activity Date: 02/28/2010**
Reward Credit Expiration Date: 08/27/2010

Tier Points are how you move to the next level of card. 4,000 Tier Points for Platinum, 11,000 for Diamond and 110,000 for 7 Stars. Tier Points always expire December 31st.

I don't know anyone who has had expired points reinstated, but it doesn't hurt to ask!