Tangochaser,
You don't have to be deployed to use the TSP. Any active duty Soldier can enroll. Until recently, there was a cap on the percentage of base pay you could contribute. Since the first of JAN, there is no cap on the percentage, but you cannot put in more than $15,000 per year.
This program has some really good benefits. The non-deployed money that you contribute is tax-deferred (don't pay interest on it until you withdraw it and gives you a break on current taxes). The money you contribute while deployed is tax-exempt (no taxes ever). There are many different funds available within the program itself, ranging from extremely secure, low interest options to more risky options with greater possible returns.
They have a great website which allows you to track returns day by day if that's your thing. Since it's an allotment - the wife doesn't have to know that you're putting this money away (unless you let her see your LES).
All in all, a pretty good program.