SectionZ from TripleTags.com seems to have found a solution for the LOADING screen lock up that occurs here and there on the PS3. He writes:
"Today both of my PS3s locked up while trying to load GTA 4.
I did the common fix which did not work. Then my brother called me. His 40 GB PS3 was having the same issue. Couple of other friends had the same issue after I called around. They could NOT play GTA 4.
And then it hit me.
WHY would this happen to me tonight after finishing the game without a hitch."