"they don't seem to exist in the real world" - information on the IBM eXFlash DIMM is here, they certainly do exist: http://www.redbooks.ibm.com/abstracts/tips1141.html?Open and also a paper on use: http://www.redbooks.ibm.com/Redbooks.nsf/RedbookAbstracts/REDP5089.html
"The things are insanely expensive(*) and may be limited to Windows." - eh?:
On the IBM Flash DIMM the Supported OS on that site shows: Microsoft Windows Server 2012 R2, Microsoft Windows Server 2012, Microsoft Windows Server 2008 R2 (Service Pack 1), Red Hat Enterprise Linux 6 Server x64 Edition (Update 5 for the x3850 X6/x3950 X6; Update 4 for the x3650 M4), SUSE Linux Enterprise Server 11 for AMD64/EM64T (Update 3), VMware vSphere 5.5 (Update 1) VMware vSphere 5.1 (Update 2)
Hardly Windows Only.
How about price? Insane? Well this technology is never going to be low cost, however lets look at this (and remembering that these are ESP (list) and no IBM client will ever pay full list price):
00FE000 200GB module has a published list price of £1638
00FE005 400GB module is list price £3000
VS a fusion (IBM Branded Fusion) card: 785GB £8855 or 1.2TB MLC mono adapter £14,265
So 6x 200GB @ £1638 = £9828 vs 1x 1.2TB £14265.
Latency...
Flash DIMM less than 5 microseconds write latency, PCIe adapters typically 10-15microseconds as they sit on the PCI-E bus.
For the right workloads, the incredibly low latency of having the Flash memory sitting right on the memory channel close to processor really does make sense, but one does need to look at:
£/GB
£/IOPS read (with equivalent block sizes)
£/IOPS write (with equivalent block sizes)
max seq read rate etc etc when comparing these kinds of technology
Of course - the next phase of Flash is coming with NVM express, the plug in PCI express disks. Think of this like taking a Fusion card, and making it a hot pluggable drive that connects to the server like existing SAS disks, but of course is direct to PCI bus, not via a SAS interface. If your interested more info here: http://nvmexpress.org/