Thanks, everyone, for your replies!
I'm am NOT much of a drinker, at all. In fact, at many parties I've attended with my DH, his friends have asked me if I'm a tee-totaler (with that tone of voice that would indicate that that is a terrible thing to be - what is up with that?). I generally don't enjoy drinking (don't like the taste of alcohol), don't want to spend the money on it and would rather have the occasional Coke (I don't drink very many soft drinks either, nor coffee.) I'm not worried about not getting enough to drink at the event, and my DH would never drink to excess at a work event, nor anywhere else for that matter. He's rather a beer snob and the selection at these things is usually poor.
When DH and I were talking about it, he said that he thought cash bars were fairly common at these events and asked me if they were all bad. My thought is that it would have been perfectly fine for the company to have made it clear on the invitation that it would be a cash bar, but for them to invite everyone to "cocktails and dinner", and then LATER spring it on us that those "cocktails" are on US was the rude thing. But perhaps I should be glad to know in advance of the event, at least.
I'm a little worried about the party. I know we don't have to stay for the whole thing, but he is a department manager, so I don't know that we'd want to bail out too early, and 5 hours seems like a long time for cocktails and dinner with no other activities.