Something like this (assuming you're using SS2K5 or above)?

DECLARE @xml XML

SELECT @xml='<Root>

<Data>

<Item ID="1" AdditionalInfo="Some info">

<SubItem ID="1">1</SubItem>

<SubItem ID="2">2</SubItem>

<SubItem ID="3">3</SubItem>

</Item>

<Item ID="2" AdditionalInfo="Some info">

<SubItem ID="1">1</SubItem>

<SubItem ID="2">2</SubItem>

</Item>

</Data>

</Root>'

SELECT

y.value('@ID[1]','INT') AS SubItemId,

y.value('.','INT') AS SubItemValue,

c.value('@ID[1]','INT') AS ItemId,

c.value('@AdditionalInfo[1]','varchar(30)') AS AdditionalInfo

FROM @xml.nodes('Root/Data/Item') T(c)

CROSS APPLY

c.nodes('SubItem') X(y)