The Keepers Registry currently has 8 participating archiving agencies:
- British Library
- CLOCKSS Archive
- Global LOCKSS Network
- HathiTrust
- Portico
- Archaeology Data Service
- National Science Library, Chinese Academy of Sciences
- e-Depot
A quick look at their descriptions suggests that some may be interested in archiving MathOverflow.
The Center for Research Libraries appears to be the main international entity for assessing and certifying the trustworthiness and reliability of major archiving agencies. Some reports can be found here.
I looked in more detail at Portico since it has a very nice description of its services. I think they would classify MathOverflow as an e-journal. Their aim is to provide digital preservation, not a backup system nor a redundant access point. So this service would be suitable for a separate read-only "e-journal" archive of MathOverflow content, not for a site mirror (even if read-only) nor for permanent storage of MathOverflow data dumps. Their publication agreement can be seen here and the terms look very reasonable to me. Their pricing model is also very reasonable: so long as MathOverflow does not make \$250k in revenues, our annual contribution would be \$250.