This move was by and large expected from the front office. Miller was, and may still be, entering the final year of his contract with a very large cap hit of $9.6 million. Lowering Miller's cap hit gives Steelers general manager Kevin Colbert greater flexibility when dealing with other players and free agency, for example signing outside linebacker Jason Worilds to a long term extension.
However it remains to be seen what form this restructuring actually took. As Miller is entering the last year of his deal, it cannot be a restructuring where some cap charge is moved further down the line, the kind of deal all Steelers fans will be familiar with after the previous few seasons.
Instead there appear to be two possibilities.
First of all, it may be a straight up pay cut. Miller is ageing, the level of his play dropped considerably and he is not long removed from a serious injury. The Front Office may have simply approached him and gave him some form of an ultimatum. This would explain why no details of the restructure have been released thus far. The Front Office may not want to embarrass a fan favorite and long time servant with a publicized pay reduction, they may instead just quietly release the details in the coming weeks when they are likely to garner less interest.
The second possibility is that the details of the contract have just not been finalized. An agreement in principle has been reached at the very least for Miller's contract, whether it be a pay cut or extension (both will result in a reduced cap hit next year). However I'm sure professional NFL contracts are incredibly complex documents, and their may be some niggling details outstanding such as performance bonuses etc.
Dale Lolley of "NFL From the Sidelines" has indicated that he believes it is a contract extension, not a pay cut that is in the works for Miller.