Is it safe to mix lye in glass? Yes, it is, but there are a boatload of caveats and limitations that go along with that. A lot of the complaints of exploding Pyrex are the result of misuse of the glassware. Pyrex in any form is not indestructible, and most people don't read the safety warnings that come with their measuring cup. There are some things that you simply should not do to Pyrex, like cooling it rapidly. But to add to that problem, Pyrex is a brand, not a material, and the materials used to make Pyrex differ.
The Pyrex kitchenware brand was divested by Corning to World Kitchen back in 1998. Up until that time, Corning was producing it's Pyrex branded kitchenware out of both borosilicate and tempered soda lime glass. Two different materials with two different set of properties.
http://www.udel.edu/chem/GlassShop/PhysicalProperties.htm
Since World Kitchen took over production of Pyrex branded consumer products, all Pyrex kitchenware is made with tempered soda lime glass. It is possible that the Pyrex measuring cup that you bought in 1975 is soda lime glass, too. Whether World Kitchen's manufacturing standards are the same as Corning's were, well, we don't know.
Corning continues to make Pyrex labware from borosilicate.
IMHO, when manufactured correctly, and when used properly (including throwing it out when it becomes scratched or otherwise compromised), both are safe for mixing lye. When I worked in a lab, we mixed strongly alkaline solutions in Pyrex labware all the time, without incident. I have used soda lime for mixing lye for many years, no problem.
(Glass of any kind is NOT suitable for the long-term storage of alkaline solutions.)
But that is a lot of "ifs" to contend with. When there are much more reliable materials, like SS and appropriate plastics, available for mixing and storing lye, why not use them?
There are two possibilities that I see for why we are hearing a lot of complaints about exploding Pyrex kitchenware. First, the rise in complaints mirrors the rise of the internet, so, whether they are justified or not, we all hear about problems with Pyrex. Second, there might be a real problem with the "new" Pyrex kitchenware. Or, it's a combination of the two. I will end by saying that the CPSC has not found fault with World Kitchen's products. I'm not sure what that means, as the CPSC is a political agency, and works in the best interest of the government, not the consumer. And I doubt that the CPSC tests Pyrex for safety in mixing lye.
Here are a couple of good online references for Pyrex, both the kitchen stuff and labware:
Wikipedia on Pyrex: Seems mostly accurate
http://en.wikipedia.org/wiki/Pyrex
Anchor-Hocking Q&A on their soda lime glass products
http://www.anchorhocking.com/Bakeware_Facts.html
Snopes runs down the soda lime/borosilicate controversy
http://www.snopes.com/food/warnings/pyrex.asp
Corning's borosilicate Pyrex lab glass safety info:
http://catalog2.corning.com/lifesciences/media/pdf/glass_care_safe_handling_RG_CI_101_REV1.pdf
