Here is an email I got from Shelley at Crucial when I emailed them awhile back about this question.
Hi Mark,
"Currently, the 512MB DDR modules we sell contain either Micron or Infineon
chips. Although we are a division of Micron, we do occasionally sell
products manufactured by other companies to meet the diverse needs of our
many different customers. At Crucial, it's our goal to bring customers the
high-quality memory products they both want and need. We've received
countless requests for 512MB PC2100 DDR, and because there has been higher
than anticipated demand from Crucial's customers, Micron cannot supply the
needed volume to fulfill all the requirements at this time. As with all
our memory, Crucial 512MB PC2100 DDR is fully-tested and qualified to meet
the most stringent requirements of the world's largest computer
manufacturers. And all Crucial DDR comes with the same lifetime warranty and
expert customer support you've come to expect when you buy memory from us.
Let me know if you have any more questions!
Thanks,
Shelley"