Scott, I love this simple post and how you’ve taken the occasion to highlight this bit of scripture.
In saying you believe this warning refers to the destruction of Jerusalem (in 70 A.D., which still lay ahead of James and his readers in the 1st C., are you also saying it is no longer applicable to readers in the 21st?
How would you stitch that together?
As in the heaven, so on the earth!

OK – so there is a potent warning that riches don’t last and are of no ultimate importance.
You aren’t thinking that this passage refers to 70AD are you?

The concrete warning of God’s historical judgment is imminent for James, and I take it to have been fulfilled in 70 AD. James warns of the perishing of riches in context of that warning; 70AD thus gives to James a concrete historical manifestation of God’s judgment against oppression and unrighteousness.
The stitch is that God judges at the historical level.

The line that you placed in bold text, “In the same way, the rich man will fade away even while he goes about his business” is very, very powerful and startling.
I wondered even as I read this text again, “What is actually happening while the rich go about their business? What is the reality that the rich are oblivious to” Hmmm. Anyway, caused me to think.

“In the same way, the rich man will fade away even while he goes about his business.”
A timely warning.
http://stockcharts.com/charts/historical/djia2000.html

John A T Robinson in his book “Redating the New Testament” (1976) concludes that James’ letter was most likely written 47-8 AD. Scot’s reference to AD 70 reflects a common, but not necessarily accurate view of the influence of the events of AD 70 upon NT writings. Notably, Robinson dates all of the NT corpus as pre AD 70. His method was based upon consideration of what, “… we should derive from the documents themselves.” rather than super imposing theories of literary development upon the NT corpus.

I just keep wondering about the phrase “the one who is rich should take pride in his low position”. Is James being sarcastic here? Is James still speaking of the rich as oppressors or could he perhaps be addressing rich converts who are perhaps being treated as outcasts in the wider community? Is there perhaps a bit of “we’re all lowly anyway” going on here meant to include a rich convert in the Kingdom calculus?
Also, I’m not sure I see shades of Temple destruction or Noah’s flood here as much as is being suggested. To me, this passage harkens more to Ecclesiastes than judgment. We have earlier passages encouraging wisdom – couldn’t this be a bit of that old wisdom being offered?

Actually, I just looked at the Greek and it appears that there is a comparison being made between low material position of the poor man and the low spiritual position of the rich man. In the kingdom, the material position is no more lasting than a wildflower while the spiritual position is what will stay with us. The rich man is going about his work unaware of the damage being done to his spiritual position. Again, I don’t really see a warning of impending destruction. More an accounting of a reality which must be seen with a spiritual eye and which can give wisdom and perspective to the oppressed believer.