While often used that way, these two are not synonymous. iPhone is the device while the SDK is for both iPhone and iPad. There can also be iPhone-specific questions that do not involve the SDK (e.g. web-development & AppStore).
The situation with the tag usage and now the renaming to iOS SDK is a bit unfortunate, but as it is now we can't differentiate questions which are about the SDK from those specific to the iPhone device.
Also of note: while we can't currently tag as
[iphone-sdk], we can tag as
[iphone-sdk-N.N] (i.e. specific SDK versions). There is also a [ipad-sdk] which would be useful as a synonym for
[iphone-sdk], but would probably confuse users if being automatically changed to