As Brad notes, moving from wage-indexing to price-indexing would result in a huge benefits cut. Had someone retiring in 2005 chosen a price-indexing system, his benefits would be 60% less than his fellow retirees, and his gas bill would go unpaid. So we should certainly oppose it on those grounds.
But one thing that I haven't seen discussed (though I'm certain it has been) is the philosophical implications of the shift. Wage-growth is a very peculiar indicator to peg payments to, and it's one that comes with a very specific message. The elderly are not a segment of society that needs the equivalent of welfare, they're not a group that we feel obligated to prop up. Instead, they were the ones whose work brought America to this point, and they deserve to share in whatever future prosperity America enjoys. It's the difference between isolating them from society as a "needy" group and rewarding them for decades of contributions. When you tack their payments to prices, Social Security becomes about keeping them alive, not including them in America's momentum. That's a real, and huge, difference, and it's one that Dems might want to include when inveighing against Bush's proposal. Using the numbers to sink the change is effective, but using the ideas to paint price-indexing's advocates as callous might prove more so.