I want to rent a house on Miami Beach for a weekend.  It needs to be
able to sleep 15 people.  It needs to be a really nice house (a la 4
star hotel).  It needs to be very close to all of the activity/cool
area of Miami Beach and it needs to be on the beach (or very close). 
We would ideally like a maid service and possibly a cook.  Our date
would be late summer or early fall.

Did you ever find a house?  I am now looking for the same, except for
a week.  I am assuming you have to use some double secret probation
realtor, as there are really no listings in the internet...

www.Renthousemiami.com is the website your looking for !

Your Welcom

Rental of a 3 to 5 star house in Miami beach requires some serious
research. I have seen this home http://www.beachvillamiami.com/ and
it's for rent ( very expensive but sleeps a lot of people ) I have
contacted this guy www.2rentit.com and he's turned me on to a few
affordable houses in the 3 star all the way to 5 star. ( from 17,000
per month all the way to 76,000 per week )

also you need to know what part of Miami Beach your rental house is
going to be in. You have Star Island, Palm Island, Hibiscus Island (
all those are off of MacArthur Highway and that road enters and exits
by 5th street ) then you have houses on Bay Road ( 24st street till
the 50's ) and Sunset Islands ( entrance on 20th street and snooty
people live on sunset island ) but Bay road has a few wonderful homes
at reasonable prices. Pine Tree Drive ( 43rd street till the 60's has
some real hot houses ) and some of those are fantastic ( there is a 9
bedroom that I have rented for 63K a month that has a tennis court and
everything)

When I have rented a house in Miami beach, I expect the following charges:
Tax = 13% 
rental rate = $$$$$
security deposit = depending on the stay and the property it's going to be a lot
Insurance = some properties I have rented required insurance. expect
about 6% of the rental contract for damage insurance with a 20,000
deductible
exit Cleaning fee = estimate about 250 per bedroom in a 4 star, 125 in
a 3 star, in a 5 star I don't know, I never booked one.
Daily maid service = $ 17.00 per hour 
evening cook = 125 to 250 per couple per evening ( you still have to
buy the food and the wine ) the cook also comes with a server or 2

Best Of Luck 

I Smile Big