@sandyf, Not sure if they still do it, bit it was the mid-90's when I starting having to pay. They sent a nice note that said I hit the threshold, and was expected to pay quarterly from that point on.
I also forgot to claim one of my many 1099's a few years ago and they sent another nice note, telling me I had probably missed one by mistake. They proactively recalculated my tax based on the missed form (it was a small additional amount) and let me know there was no penalty due if I paid within 30 days.
I have heard many scary tales about the IRS but my personal experience with them has always been really positive.....