It really depends on the situation and how casual the affair is. According to Ms. Manners, at a formal, sit down dinner, the hostess has already presumably chosen a wine to compliment the meal. Likewise with flowers and dessert. It is not bad manners for her to put away wine or candy or food or flowers for later. It could just as easily be argued that it is bad manners for guests to "redo" the hostess dinner plans at the last minute when she may have carefully planned it all out to the last detail.
If it is a casual affair or a potluck, then that is different, and the guest would not be remiss for asking if he could open the wine now for everyone to taste.
Local custom should also be considered. In "high society", it is not the custom to bring hostess gifts at all for the same reason I mentioned above. Cards, flowers, gifts etc are sent after the dinner is over as a proper thank you. It is a very middle class custom to bring a hostess gift and as this thread demonstrates, they can create a lot of misunderstandings and hurt feelings if every one is not aware of, or shares the same view of the custom.
Unless, you were specifically asked to bring an item or told that it was pot luck or BYOB, there is just no reason to be offended when your wine or other food is not served at the party. It is up to the hostess discretion. However, it is always the height of bad manners to accept someone's hospitality and then trash them and it behind their backs after you leave!
Cog