The letter, to the president of Museum Tours, claims its site contravenes US patent no. 5,933,841 called the "Structured Document Browser" and has given it a chance to license the patents.
SBC is using a "Royalty Equation" to figure out how much Museum Tours might own it by allegedly infringing its browser patent.
The letter, apparently from a Mr or Ms Harlie D. Frost, also helpfully includes the kind of royalties Museum Tours - which you can find here, may have to fork out. It's quite cheap if you're a weeny site.
And Museum Tours has a link to the letter which came as part of a 40 page package - it doesn't say how much that weighs, here.
In the paragraphs ahead of the letter, the site claims that m'learned fiends are going after the "small folk" first.
Here's that patent on the US government web site.
And here are some earlier patents. ยต
* SINCE WE FILED the above, the relevant letter seems to have been replaced with images, which you can click from via the directory.