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You're probably a little light on the I fund. The I Fund is good for portfolio diversification because it non-correlates with the C&S Funds, i.e. often moves in the opposite direction and tends to smooth out your returns over time. Many experts recommend a 25% allocation to non-U.S. equities.
The Vanguard 2045 fund (designed for a person retiring in 2045) currently holds 70.8% total stock market (C&S Funds), 17.8 European/Pacific stock (I Fund), and 11% bonds (F Fund), and .4% cash. To date, it has returned 14.41% in 2004.
Note: my comments only apply to abuy-and-hold portfolio.