There are still some index arb opportunities, but in reality you are arbing funding levels, margin financing etc. It gets especially hairy on quanto index futures etc where you are trying to arb convexity. To make the long story short - it's not worth the effort unless you have a balance sheet. Even then, there are some significant risks.
There are no free lunches. There are some cheap lunches. Even with cheap lunches, however, you frequently will discover that it's not a potato but a large rock.