Well it's debatable wether or not he entered the portal or not.
I'll borrow this from another thread I saw a while back. Slipspace works like a hallway.
You enter in through a doorway, travel down the hallway, and come out another doorway. Apparently there a currents and eddies (like the sea) that can influence the jumps. The Forerunners had a extremely strong grasp of how it worked. The portal from Earth to the Ark, took a tiny amount of time compared to what it would have taken if using a Human or Covenant jump.
Anyway, The Chief could have either:
a) gone through successfully and ended up at Earth.
b) got left behind and died.
c) got left behind and survived.
d) got taken into the "hallway" and then broke off staying in Slipspace.
e) got taken into the "hallway", then broke off and finally reverted to Realspace.
f) got blasted into the past/future.
Now to take out the impossible ones:
a) didn't happen.
b) didn't happen.
c) unlikely.
d) plausible.
e) unlikely.
f) unlikely.
Finally we have these:
c) How did John survive? We don't know. It's probably not true, but it's a possibility.
d) How did the ship split in two? And what's that planet? I don't know about splitting in two, but I think that the planet is another Micro Diosphere that is shaped like a planetoid instead of a bowl shape (like the one in The Ghost of Onyx.
e) First off, how would he revert to Realspace? Virtually impossible.
f) There is a time/space continuum present inside of Slipspace, however it seems highly unlikely that they would be shunted through time. Consider the explosion though, it could have done something to the rupture making them somehow get pushed into the past/future.
All in all, I believe they are still in Slipspace and came to another micro Dyonsphere. However, there are several other possibilities present, and we cannot rule them out. Though, all those threads presenting evidence because of: Slipspace ruptures, blue lights, same explosion craters, etc. prove nothing.
Hope you got some questions answered and learned a thing or two.
~Hunter
[Edited on 06.14.2009 11:27 AM PDT]