There's only been a limited amount of tv's that have built in slingplayer capabilities. A JVC model and some of the Sony tv's that have googletv. There was a few LG models, but they upgraded the honeycomb O.S and that did away with flash which did away with the ability to use the slingplayer. If you don't have one of those TV's, that leaves you needing an external connected device. Right now, the wdtv live and play boxes aren't working right with the slingbox's. They're dropping the slingbox connection after a short while, so I'd stay clear of them for now. The googletv boxes continue to function, albeit with a few tweaks to make it work with the direct slingplayer weblink, but they are officially no longer being supported and are on life support, so that might be an issue down the road. The only connected device I see that sling is putting any resources into is the roku boxes for use with the iphone and ipad. A recent app upgrade has allowed your iphone or ipad to be put into standby mode and still stream to your big screen tv. Before, the slingplayer app had to be always on the screen. Now it can run in the background. I've tried both versions and they work pretty well with the roku box.
so, you might want to consider a roku box since you've already got the ipad and iphone to make it work with your tv, because unless your smarttv is sling connected, it won't work.