I have done this using this code.
http://support.microsoft.com/kb/892730

You can try the method described in this blog post:
http://chrissyblanco.blogspot.com/2006/07/infopath-2007-file-attachment-control.html

The translation to VB.NET would look something like the following, where
base64encodedstring is the base64 encoded string for the attachment:

Dim attachmentNodeBytes() As Byte =
Convert.FromBase64String(base64encodedstring)
Dim fnLength As Integer = attachmentNodeBytes(20) * 2
Dim fnBytes() As Byte
ReDim fnBytes(fnLength)

Dim i As Integer
For i = 0 To fnLength
   fnBytes(i) = attachmentNodeBytes(24 + i)
Next

Dim charFileName As Char() = UnicodeEncoding.Unicode.GetChars(fnBytes)
Dim fileName As String = New String(charFileName)
fileName = fileName.Substring(0, fileName.Length - 1)

Dim fileContents() As Byte
ReDim fileContents(attachmentNodeBytes.Length - (24 + fnLength))

For i = 0 To fileContents.Length - 2
   fileContents(i) = attachmentNodeBytes(24 + fnLength + i)
Next

Dim fs As FileStream = New FileStream("C:\" & fileName, FileMode.Create)
fs.Write(fileContents, 0, fileContents.Length)
fs.Close()

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S.Y.M. Wong-A-Ton