I found this on an etiquette website:
"Guests invited to the wedding have an obligation to send a gift, whether they are attending or not. There are few exceptions. If you live far away from where the wedding will take place and have been out of touch with the couple for several years, and are not planning to attend the wedding, there's no need to send a gift."
I think this also holds true for those that are invited to the wedding but not the reception, though personally I think it's quite rude to send an invite to one and not the other, unless it's a church wedding, which at least for catholics are public (so yeah, people can walk in off the street if they wanted).
If you are only attending the wedding, and you do choose to send a gift, it should be sent ahead of time. Traditionally, no gifts should be brought to the reception, either, since it would appear the couple and/or the giver was showcasing the gift, but I think most people don't really follow that anymore, at least if they're only giving something small or some cash.
You can probably find more info by searching for emily post or other such websites.
Of course, there should be no mention of gifts (not even to say they don't want any), or cash, or registry websites on wedding or reception invites. Most people ignore this, though, and one of my friends sent out her own shower invites.. but I love her so I overlooked it. :D