You will not ever find your self in 99% of hunting scenarios in America where you will not have enough rifle with a .30-06 loaded with the right bullet for the game you intend to hunt.
You will always find ammunition available, and the handloading versatility of the .30-06 is unmatched.
As a long range cartridge, the .30-06 in a suitable rifle, with bullets that are suitably constructed to reliably expand at the anticipated velocity, and in the hands of a capable marksman is an effective medium, medium/large game killer well beyond 900 yards.
There are flatter shooting, harder hitting cartridges, but there are none more versatile than the .30-06.
With all of the flame throwing that is sure to follow, here is a quote from someone who does such things professionally, and has studied the effects on game at long range with the .30-06. I will include the link to his page for informational purposes, and so as not to take credit for his work.
"The 178 grain A-Max is a hard hitting bullet and can be employed for all-round work on game weighing up to 150kg or so, again, with an understanding towards limitations and shot placement at close ranges on larger animals. On game weighing less than 90kg, the A-Max can be used at woods ranges without fuss regarding penetration. The strengths of this bullet do of course occur at long range and the 178 grain A-Max fragments in a reliable manner, right down to 1400fps (900 yards). The sound barrier and potential associated bullet yaw occurs at around 1200 yards from an MV of 2800fps. BC is .495."
http://www.ballisticstudies.com/Knowledgebase/.30-06+Springfield.html