I just researched this by the etiguette gurus. What I keep reading over and over again is that baby showers are not about greed, but about welcoming a new life into the world. However, the mother should not throw her own shower. I think that's a given whether it's the 1st, 2nd or 10th baby.
Here's what Emily Post has to say:
For the week of June 20, 2005
Q. Is it proper etiquette for an expectant mother to host a baby shower for her 2nd baby?
A. While it is never appropriate for someone to host a shower (baby or bridal) for themselves,
it is perfectly fine to throw a baby shower for a mothers second or third baby, as long as the guest list is limited to close relatives and very close friends and/or guests who did not attend a shower for the first child. It is an especially nice event for the mother-to-be if several years have passed since the last baby was born, since the parents will have fewer hand-me-downs for the new arrival. Location can also play a part. When a growing family has moved to another city or town, it makes sense for their new friends to throw a shower, regardless of how many children the parents have.
From stork.net
Showers for second (or more) babies
Q. Is it improper to have a baby shower for your second baby, especially if the sex is different?
A.
It is never improper to have a baby shower. A shower is so much more than gifts. It is an opportunity to gather family and friends together to celebrate the beginning of a new life. The birth of a baby is a joyous occasion, and it is always acceptable to celebrate it. If other family members or friends are concerned about it being improper to have a baby shower for a second (or more) child, there are ways to take the focus off the gifts. If the second child is a different sex than the first, it would be fun to host a "It's a Girl" or "It's a Boy" party, where guests bring gender specific gifts. Or, if the mom-to-be truly does not need anything for the new baby, forego gifts altogether and just gather to celebrate. Another option is to host a casserole shower or a pampering mom shower.
And other links that say its okay:
http://entertaining.about.com/cs/showers/a/secondbabyshow.htm
http://www.mothering.com/discussions/archive/index.php/t-53070.html
http://www.tellinitlikeitis.net/200...wer-etiquette-is-a-second-baby-shower-ok.html