We use the Raspberry Pi Compute Module, i.e. a special, stripped-down version of the Raspberry Pi which fits on a 200 pin SO-DIMM module. This version does not use an SD card but has a 4 GByte eMMC on-board. (It would be possible to use an SD card in addition to the eMMC but we don't make use of that.)

Of course the eMMC is still based on flash, which has a limited lifetime. So we use a customized version of Raspbian which has swap disabled by default (can be enabled if you need it) and tunes the syslog configuration to reduce the amount of log data written. It's also possible to use a tmpfs-backed /var/log (can be enabled with a mouse click or a simple command). That way we try to prolong the lifetime of the eMMC as much as possible.

We've also been in talks with the Foundation about this and they have asserted that the eMMC is of good quality and should last for the typical lifetime of a PLC (which can be decades).

We are in a very close contact to the Raspi Team and I can assure you that most of those "SD card issues" is pure rumor and urban legend. All known issues could be reduced to the fact that some people bought cheap cards and supposed them to work for ages even when treating them like you would never do with your PC. Raspi team's Sw developers told me they would switch off power without shutting the system down dozens of times per day and never ever had an SD card crash for years now. So please do not spread the rumor without any substantial first hand information. Just because someone did write something somewhere in the internet we can't judge about a technology. Having this said I would like to add the information that RevPi Core does not use any Sd card. nPlease refer to the Compute Module 1 information in the internet to get this Version of the Raspi explained.

I can't edit my post any more, but want to acknowledge this answer. Thanks!