There is no way to transfer or convert the SlingPlayer application from the Android platform to an iPhone device. Transferring or extending your SlingPlayer registration depends on where you bought it, have a look at this article,
I'm having a similar problem, I just switched my Blackberry Bold to an Android Galaxy SII yesterday and I bought my copy of slingplayer from the Sling website, so I have the registration number. But when I got to Android Market, they do not offer a trial version of it, you have to buy it. So how can I transfer my registration and get a copy of slingplayer for android without paying again.