The Treasury recently announced that as of April 7th minimum investment amount for government bonds will be lowered to $100 (previously $1,000). This includes all Treasury marketable bills, notes, bonds and Treasury Inflation-Protected Securities (TIPS). Thanks for the e-mails.
This means you will be able to build a weekly Treasury Bill ladder for as little as $400. Unfortunately, right now yields are so low that I have no reason to bother. The last T-Bill auction resulted in an investment rate of only 0.527% for the 4-week T-Bill, 1.337% for the 6-month T-Bill, and 2.045% for the 2-year T-Note.
Even TIPS are so much in demand that some of them have been trading with a negative real yield. If you are interested in inflation protection, especially for mid-term periods like 5 years, it may be better to simply buy a Series I Savings Bond, which right now has a real yield (fixed rate) of 1.2% through April 30, 2008. You can buy those already for as little as $25 via TreasuryDirect.gov. (There is a purchase limit of $5,000 online and $5,000 paper, although people have reported being able to buy more online.)