Several months ago, we received an e-mail from a bride which included a link to her wedding website. The site contained details on lodging arrangements for out of town guests, as well as links to places where she had registered. We sent a gift to the couple and waited for our invitation to arrive in the mail. In the meantime, e-mails came from the bride's parents and other guests about wedding events, but with three weeks to go, no wedding invitation was received. This puts us in a awkward spot because we don't know if the invitation was lost in the mail or if the couple decided not to invite us at the last minute. The bride called recently to ask if we had received her thank you note for the gift, but did not ask about the invitation. She ended the conversation by saying "See you soon" (we live on opposite coasts). What is the polite thing to do?
Normally, it wouldn't be polite to ask because we don't want it to appear as if we are asking for inclusion or to put anyone in a difficult situation. However, under these circumstances, and especially since you know the parents, I would ask the parents about the invitation. It may be best to approach it lightly by saying that you weren't sure about the invitation and didn't want to put anyone in a difficult situation, but... .
If they didn't invite you, it is a major faux pas on their part. They may not know it though. Hopefully this is not the case.