I realize this thread is a bit old, but I just came across it.
First, I realize that none of the old "Mobile" or "Portable" identifiers are required anymore, but I wanted to point out that there seems to be a misunderstanding here about what was required "back in the day." If I missed where someone else pointed this out, I apologize.
I dug out my 1978 License Manual to confirm this, so as not to rely on my foggy memory.
Definitions: Portable Operation: Radiocommunication conducted from a specific geographical location other than that shown on the station license.
Mobile operation: Radiocommunication conducted while in motion or during halts at unspecified locations.
Based on these definitions, and my memory of how we used to do these things, the "Portable" designator is NOT used just because you are using a portable radio / HT. In most cases, the HT user who is say, out walking his dog, would still use the "Mobile" designator, because he is in motion or stopped at a random location. If you were in your own house, at the location specified on your license, no designator is needed even if using the HT.
The "Portable" designator is properly used when you have a station set up at a specific but perhaps temporary location. A good example would be a vacation home where you have a station that you only use a few weeks or months of the year. This is the type of situation where you would use the "Portable" designator.
Update: In re-reading this thread, I see that a couple of others did point this out correctly. But it was fun digging out the old license manual anyway.