So, this one has actually come up in a real, honest to Kermit, game.
Rhino A Rams Deff Dredd B. The resultant collision immobilizes both vehicles, thus leaving them in contact ( as per the rules on page 69 of the Big Black Book: "[. . .] if the ramming tank comes into contact with another vehicle, the collision is resolved [. . .]"). In the Assault phase, the question arises whether the Deff Dredd now gets to attack the Rhino in Close Combat. Coming to the conclusion that the only way this would happen was if it had assaulted the Rhino in a previous turn, we ruled no. I stick by this judgment. See page 63 "Successive Turns" for the rules applicable here.
In the Deff Dredd's assault phase, however, something seems weird. Finding no justification for anything else, we ruled that no attacks could be made, since the Dredd couldn't assault.
I maintain that this is an accurate reading of the rules as they are. I do, however, feel that there is a case to be made for the Dredd being allowed to assault. See, there's not a single sentence (as far as I can find, and I have looked) disallowing an assault merely because the model cannot move. Oh, it will most assuredly fail, since at almost all times models must remain at least 1" apart, so an assault where you can't move will leave you with no base contact.
But, as has been established, the Dredd is in base contact, so could declare an assault, not move a bit, and end up in base contact, thus being allowed to fight. Seeing how ridiculous it feels that the enraged Ork would just sit back and enjoy the view with a perfectly fine target right under its nose, I'm going to go with this second interpretation.
And that's it for now. Happy Darwin Day.