On July 1, 2010, all the money in the L 2010 fund will disappear. In fact, the entire L 2010 fund will disappear as well.
On that date the fund mixtures in the L 2010 fund will mirror those in the L Income fund and the two funds will become one.
The TSP will introduce a new fund, L 2050, around the time that the L 2010 fund merges into the L Income fund.
All of the L funds (except L Income) gradually adjust to a more conservative allocation. Adjustments are made every calendar quarter. Ultimately, all funds turn into the L Income fund. For example in July 2020, the L 2020 fund will disappear and the L 2060 fund will be introduced.
The TSP introduced the L funds as a way for TSP account holders to have a diversified portfolio that gradually adjusted as the account holder got closer to the date he or she would need the money. They bill the funds as "putting your investments on cruise control". More information on the L funds can be found at http://www.tsp.gov.