I attend several white tie events. I think that white tie calls for a real ball gown. Frankly, that Jil Sander dress (previous link) was hideous, and $600+ for polyester? No thanks!
While it's true that your shoulders should be covered by a shawl/wrap initially, you can certainly remove your wrap once inside and definitely for dancing. I was invited to the Vienna Opera Ball several years ago, which is a strictly white tie event, and many ladies wore bare shoulders. This is still the case:
http://www.kleinezeitung.at/freizeit/events/baelle/2691039/55-wiener-opernball-feierlich-eroeffnet.story
See photo and article from Austrian newspaper.
Here is an old picture from 1954 depicting Her Majesty Queen Elizabeth II. at a white tie event in Australia. I can assure you that such events have a very strict protocol - much stricter than anything you will ever see in the United States - and even there you saw some bare shoulders.
http://www.nma.gov.au/exhibitions/royal_romance/crowns_and_gowns_2?result_6095_result_page=2
Even Her Majesty shows her shoulders.
Now, as far as the gown is concerned, I would pick either a ball gown or A-line silhouette. How much time do you have? How about something like this?
http://www.saksfifthavenue.com/main/ProductDetail.jsp?PRODUCT<>prd_id=845524446413382&site_refer=AFF001&siteID=Hy3bqNL2jtQ-Nevo0t_GvjpYrR2kDdhZSw&LScreativeid=1&LSlinkid=10&LSoid=246445&LSsid=Hy3bqNL2jtQ
The fabric is silk organza, which is beautiful and has a nice crisp feel. It comes with a wrap in the same fabric. Pair it with a simple necklace and earrings, and wear off-white silk or kid leather gloves:
http://www.lordandtaylor.com/eng/JewelryAccessories-gloves-Opera_Length_Silk_Charmeuse_Gloves-lordandtaylor/161806
You can also try to get these on eBay for much less.
If you prefer a more substantial fabric (e.g., silk taffeta or silk dupioni) why not something like this?
http://www.thebridalshop.com/montage/110904.htm (it's polyester taffeta, but still pretty)
Or something in chiffon?
http://www.edressit.com/edressit-2012-new-simple-elegant-evening-dress_p1994.html
BTW, this site has some lovely dresses. I've ordered in the past, and they do a nice job, but it takes a few weeks.
Cheers!