achowe, Why won't Npeeters query work for you? That's a nice little piece of code, deletes all records except the one with the maximum date value.
DELETE t1
FROM MyTable t1
JOIN MyTable t2 ON t1.Unique_ID = t2.Unique_ID AND t1.DateField < t2.DateField
And, if you have an identity column and you insert earliest dates first then you should use that instead. SQL handles integers a lot better than dates.
--something like...
DELETE t1
FROM MyTable t1
LEFT JOIN
(Select max(t2.Identity_ID) Identity_ID
From MyTable t2 (nolock)
Group by t2.Unique_ID) t2 on t2.Identity_ID = t1.Identity_ID
Where t2.Unique_ID is null
Signature is NULL