If you want to broaden your US stock market exposure but stick with an index fund, take a look at the TIAA-CREF Equity Index Fund. Although it is labeled a large cap blend, the index it follows is the Russell 3000. That would give you some exposure to smaller companies, if you don't have that somewhere else and would like to include it. The expenses are higher than Vanguard. You could also consider the TIAA-CREF small cap blend index fund and the Vanguard mid-cap index if you decide to broaden your equity exposure. In your shoes, I would consider doing this with a small part of the 63 percent, but again, the S&P index fund is a fine choice, and it depends on what you have in your other accounts.
I really have to commend the State of Connecticut for requiring ING to offer decent quality funds. Other states, counties, and cities should require the same for their 457 plans.