Where did you "hear" this? You make it sound like these are two different commands in apt-get. Is that what you mean? Aptitude has been using new command names: full-upgrade and safe-upgrade for awhile now. I could imagine apt-get switching names as well, but I'm using Lenny, so I don't know about newer versions of apt-get.
In any case, the reason for Aptitude's name-switch, I think, is that dist-grade was always a bit confusing. You don't only use that command to upgrade from one branch of Debian to another. The name-change is clearer (even if it does require more typing - but that's why the Unix gods invented aliases).
"We have not been faced with the need to satisfy someone else's requirements, and for this freedom we are grateful."
Dennis Ritchie and Ken Thompson, The UNIX Time-Sharing System