Get Trimmed List Of Documents From Document Library (Professional Edition) So I'm building a module where I have to add commenting ability to the built-in Document library feature. I'm trying to find where I pull the documents from. I also want the list security-trimmed. I'm really hoping there's an API and I don't have to build a manual solution that breaks on an update of the document library...
Any help on where to get this information?

You may be able to use the DNN File Picker control, or re-purpose some of it's logic.
The source code available on Codeplex has examples of retrieving folders for a user and an example of retrieving files for a folder (for a user):
http://dotnetnuke.codeplex.com/SourceControl/changeset/view/58672#612642
Private Sub LoadFolders()
    cboFolders.Items.Clear()

    'Add Personal Folder
    If UsePersonalFolder Then
Dim userFolder As String = FileSystemUtils.GetUserFolderPath(UserController.GetCurrentUserInfo().UserID)
Dim userFolderItem As ListItem = cboFolders.Items.FindByValue(userFolder)
If userFolderItem IsNot Nothing Then
userFolderItem.Text = Utilities.GetLocalizedString("MyFolder")
Else
'Add Dummy Folder
cboFolders.Items.Add(New ListItem(Utilities.GetLocalizedString("MyFolder"), userFolder))
End If
    Else
Dim folders As ArrayList = FileSystemUtils.GetFoldersByUser(PortalId, ShowSecure, ShowDatabase, Permissions)
For Each folder As FolderInfo In folders
Dim folderItem As New ListItem
If folder.FolderPath = Null.NullString Then
folderItem.Text = Utilities.GetLocalizedString("PortalRoot")
Else
folderItem.Text = folder.DisplayPath
End If
folderItem.Value = folder.FolderPath
cboFolders.Items.Add(folderItem)
Next
    End If

End Sub

and
Private Function GetFileList(ByVal NoneSpecified As Boolean, ByVal Folder As String) As ArrayList
    Dim fileList As ArrayList

    If IsHost Then
fileList = Globals.GetFileList(Null.NullInteger, FileFilter, NoneSpecified, cboFolders.SelectedItem.Value)
    Else
fileList = Globals.GetFileList(PortalId, FileFilter, NoneSpecified, cboFolders.SelectedItem.Value)
    End If

    Return fileList
End Function