According to the IRS advisory to tax preparers dated December 27th:
A prepayment of anticipated real property taxes that have not been assessed prior to 2018 are not deductible in 2017. State or local law determines whether and when a property tax is assessed, which is generally when the taxpayer becomes liable for the property tax imposed.
According to my county tax office my property was assessed (and revised) in 2010, and will likely not be reassessed for several years (provided I don't do something to improve it's value).
However, the IRS comment that the "assessment" is generally "when the taxpayer becomes liable" suggests that they (the IRS) are conflating "assessment" with billing or invoicing.
On the one hand, as my property was "assessed" in 2010 - it would appear that I could pay my tax bill which will come to me in March 2018 prior to Dec 31 2017 (effectively prepaying) and use that as a 2017 deduction.
On the other hand if the IRS (sometime in 2018) says 'Oh, we really meant tax billing, not assessment' then that deduction taken in 2017 would fail in audit.
So exactly what does the IRS mean by "assessment"