My 401(k) is invested about 50% in one of those target date retirement funds and about 50% in a mix of other stock/bond funds (I'd put it all in the target date fund but I want a more aggressive mix than they use, and I'd put it all in other funds but my selection is quite limited.) Overall it's about 60% US stocks, 25% foreign stocks, 10% bonds, and the rest is cash/short-term and "other".
YTD: +9.9%
Q107 (1/1-3/31): +2.5%
Q406 (10/1-12/31): +6.8%
11/1-4/30 (~last 6mos): +10.6%
Those are TWRR values from Fidelity -- they're straight-up returns, not an approximated APR or anything like that. The overall market has been quite good lately.
Market timing is extremely difficult even if you're a financial expert. If you have a sizable chunk of cash to invest with right away and you're concerned about buying in right before a big drop, buy into investments over the course of a few weeks or months, splitting your nest egg into equal purchases. Dollar-Cost Averaging (a fancy name for "investing the same amount of money at regular intervals") works well, and fits nicely for most people who would be investing into a 401(k) or taking a chunk of their monthly budget and saving it.