"collo serafino" turns up a wide variety of necks, not just the sedate
grandad neck (a name which is not commonly used among most English speakers) See here for some examples of "serafino":
http://cgi.ebay.it/ws/eBayISAPI.dll?ViewItem&item=2702607679...
http://www.twenga.it/marca/2182-Soft-Grey.html
http://www.laredoute.it/IT/it/acquisto-pulletco-donna-catego...
http://www.laredoute.it/IT/it/vendita-camicetta-iridescente-...
Some have band collars, some have no collar, some have shirt collars.
What they do have in common is the opening - a central front (partial) placket.

http://en.wikipedia.org/wiki/Placket
http://www.google.it/search?hl=it&q=pajama "partial front op...
http://www.amazon.com/P-Jamas-Smocked-Short-Sleeve-Nightgown...

I think that most consumers would understand most easily what it is of you use a description such as "round neck"/ "high round neck" and then use "front (button) opening" or "partial front opening" or "front placket opening". And of course it would be best if you saw a picture first !

HTH

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Note added at 6 hrs (2008-10-18 21:22:20 GMT)
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nel film "Serafino" (P. Germi, Italia 1968), il Molleggiato (Adriano Celentano)in versione li pecuri sfoggiava maglie di lana senza collo. di qui l'espressione "collo Serafino" o "Serafino" tout court, diventata terminus technicus del ramo biancheria.

http://caporalereyes.splinder.com/post/14430269/Cose che not...