Since Saturday morning the NTP Pool server have had trouble routing to some servers in the pool system (about 5%). This was too few to trigger the "help help, something's wrong!" alerts; so thanks to those of you who sent in tickets!
I opened a support with our provider and hopefully the issue will be resolved shortly.
The monitoring server is on the same network as www.pool.ntp.org (in AS 7012) if you have trouble and want to check traceroutes or BGP information from your end.
Due to the distributed nature of the pool system, time serving for clients should not be affected.
Update - One of the "upstream" providers were dropping a seemingly random group of networks. Phyber took down the BGP session to them until it's sorted and all is well again as of around 5pm pacific time.