Does this work for you? It returns 30-8-2010 13:21:42 at the moment on SQL Server 2005:
select dateadd(s, convert(bigint, 1283174502729) / 1000, convert(datetime, '1-1-1970 00:00:00'))
I've divided by 1000 because the dateadd function won't work with a number that large. So you do lose a little precision, but it is much simpler to use.
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