I submitted a case to have this feature enabled. They have enabled the feature. Prior to the feature being enabled the is-modified field was null.
Request: https://na15.salesforce.com/services/data/v27.0/chatter/feeds/news/me?sort=CreatedDateDesc
Response: {"feedItemsUrl":"/services/data/v27.0/chatter/feeds/news/005i0000000qJX7AAM/feed-items","isModifiedUrl":"/services/data/v27.0/chatter/feeds/news/005i0000000qJX7AAM/is-modified?since=2%3A1368707238000"}
Request:
https://na15.salesforce.com/services/data/v27.0/chatter/feeds/news/005i0000000qJX7AAM/is-modified?si...
Response:
[
{
"message" : "Invalid feed modified token: 2%3A1368707238000",
"errorCode" : "INVALID_FIELD"
}
]
