Becareful what you wish for. I also own an iphone 3g and would love for Sling to be on it as an app.
However, judging from using my BB Bold with Sling (it's wonderful with the Bold's high resolution) network conditions will be tricky.
I was in Knoxville, TN (pop. around 500K) and using my BB Bold with Sling on 3g during Super Bowl since I was traveling. Perfect conditions (maybe less users in Knoxville, TN).
The minute I hit Orlando (bigger area population wise), I launch Sling on my BB Bold and kept on getting interruptions.
I'm afraid if I travel to a much larger area like my home area of Washington DC or even up to NYC for the weekend, I would be very limited to what I could watch on sling with the iphone 3g data.
Sling is very intensive and if it's available on the iPhone 3g, it will degrade their network.