The [Keepers Registry](http://thekeepers.org/) currently has 8 participating archiving agencies:

* [British Library](http://www.bl.uk/)
* [CLOCKSS Archive](http://www.clockss.org/)
* [Global LOCKSS Network](http://www.lockss.org/)
* [HathiTrust](http://www.hathitrust.org/)
* [Portico](http://www.portico.org/)
* [Archaeology Data Service](http://archaeologydataservice.ac.uk/)
* [National Science Library, Chinese Academy of Sciences](http://english.las.cas.cn/)
* [e-Depot](http://www.kb.nl/en/expertise/e-depot-and-digital-preservation)

A quick look at [their descriptions](http://thekeepers.org/thekeepers/keepers.asp?action=agencies) suggests that some may be interested in archiving MathOverflow.

The [Center for Research Libraries](http://www.crl.edu/archiving-preservation/) 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](http://www.crl.edu/archiving-preservation/digital-archives/certification-and-assessment-digital-repositories).

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I looked in more detail at [Portico]() since it has a very nice description of its [services](http://www.portico.org/digital-preservation/services). I think they would classify MathOverflow as an e-journal. Their aim is to provide [digital preservation](http://www.portico.org/digital-preservation/services/preservation-approach), 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](http://www.portico.org/digital-preservation/join-portico/for-publishers) 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.