ReadWriteWeb posted a short article today, The iPad Not a Savior of Magazines, as Digital Sales Continue to Fall. Looking at the current sales trend is pretty silly, however. Of course you're going to see a big drop-off. Early adopters are eager to check out The Future, but people aren't going to continue to pay newsstand prices for digital editions of magazines, especially if they're already paying for a print subscription.
I don't think we'll see much improvement until both Apple and the publishers get over themselves and come to an agreement on subscription pricing. At issue is how much access the publishers get to their subscribers' data. Traditionally, what keeps print publications alive is their ability to resell their subscribers' information to advertisers. But Apple isn't about to give that up. So either publishers need to accept that they're going to have to give up some control in order to play in Apple's ecosystem, or Apple needs to loosen its grip a bit (less likely).
Still, I'd wager that in 5 years, digital subscription sales will surpass print.