I bet it is because 1) your phone was one of the lucky ones that that got bumped up to nd 2) you have a lot of other stuff running in the background, and 3) you're streaming that music on a high speed (4G) account.
ICS makes true use of the dual cores in the CPU of your phone, so all that stuff running in the background can run even while foreground apps (like your music program of choice) runs as well. This means faster battery drain.
For a test, try a FDR and only reinstall Pandora, not other apps, and see if it still kills your battery while playing. If it does, you might need to consider rooting your phone so you can control processor clocks (I had to do the same with my OG DROID - Pandora + Nav = massive power drain, one that even that 1.0 A car charger could not keep up with). Or else invest in a battery saving app from the market to see if it helps.
I'd also spend the money on an extended battery (buy new, not used off of eBay).