i wouldnt say its rude, but think about if you were the average clueless wedding guest. (FI and i just had this discussion last night actually!) sending out your invites so far in advance, people are likely to forget about them and lose them, toss them out by accident, etc etc, spend all their money in advance. you are better served to send out a STD, and these can be done very cheaply on your own, referring people to your wedding website. this is what we are planning to do, and including a small business card sized enclosure with the STD that due to the nature of a destination wedding, we need a preliminary headcount, and we will be asking people to go to our website and add their name to a list on a special part of the site if they think they will either definitely come or maybe come. our STDs also say "formal invitation to follow" -- hope this helps a little!