This is a topic that has been discussed several times before in regards to public figures, impersonating specific users on the site, and profile pictures by themselves.
In general, the attitude is that you can use most profile pictures of public figures, and even name yourself after them, but you may draw more attention from moderators by doing so. Filling out your public profile as if you were that person is usually a step too far.
Copying another Stack Exchange user's name and avatar is generally considered an attempt at impersonation, and will typically be rolled back by moderators. Filling out your profile to match the other person is certainly an attempt at impersonation, and will almost always trigger a profile rollback and stern warning.
It's a question of how likely it is that people would in any way believe the person who is posting is the figure from their profile. I think it's pretty easy to determine that none of the accounts named "Steve Jobs" on Stack Overflow are the person we think of (for obvious reasons), but this is a large enough network that it's not inconceivable that someone is independently named Steve Jobs and is using the site.
The linked case was a trickier one in that someone was using a photo they took of someone else in their profile (generally OK, plenty of people use photos of significant others or their children as avatars in a positive way), but then they also gave their profile the name of this person (starting to get more problematic), and then filled out the rest of the profile as if they were them (that's stepping over the line). Again, it's a matter of degree, and each case will require personal judgment.
