It's a form of ramming, yes, but it occurs unintentionally. I remember the ship one, now. It was mentioned in one of the books I think. I was thinking along the lines of what Milksahe said for a long time, then I thought that that might not be right.
I always figured the answer was nothing. Only objects large enough to exert gravity into hyperspace can cause damage to whatnot to a ship, or some bullshit like that. Who the hell knows. It's all made up anyway so you're likely to get about sixteen different answers all with justification in canon. It's kind of like the Bible.
Elite, highly trained Imperials are not likely to ram a vessel- while crazed, rag-tag Coa.. I mean GLF terrorists would be much more likley to ram.
Not that I suggest ramming be considered a tactic to use often with living crews... but Robotram ships do exist in canon (the sucky Correllian trilogy if I remember correctly)