Winter ‘18 began to make appearances on existing sandbox instances around the 8th September.
A previous issue with Winter ‘18 and iOS that was found by MobileCaddy was escalated to Salesforce through our ISV team. Further discussions were had with senior Salesforce engineers, and these resulted in a Salesforce patch being applied to pre-release orgs on the 12th September. We re-ran our daily tests following the patch and can confirm this
There is a new open issue with Winter ‘18 preview and sandbox instances affecting Android devices affecting community licenses. This is being monitored and we have fixes for container apps currently within QA.
Timings for Winter ‘18 can be seen in this graphic, released by Salesforce.
From time-to-time Salesforce undertakes a maintenance operation called an Instance Refresh. This is performed for various reasons, but primarily to maintain good performance. As part of this, your org may be moved to a new data centre. More information on this, and steps that might be needed, can be found in this Salesforce Help Document.
* Correct at time of writing.