Dreamweaver doesn't have a dedicated interface for viewing TODO items. However, it does have what it calls Design Notes. These notes are stored in *.mno files that are within _notes folders within the folder of the file that has a design note attached to it. These files and folders are only viewable within Dreamweaver if you have Show Hidden files enabled, which you can do by accessing the Files panel options menu on the upper right by clicking on it, then selecting View -> Show hidden files.
You have to turn them on in the site definition, although by default, a Dreamweaver site will have them turned on. The following steps are for Dreamweaver CS4:
Site -> Manage Sites..., Select the site, click Edit, select the Design notes category, check maintain design notes. Next go to the File View Columns category and make sure that the Notes column is displayed. Click OK then Done.
You can also opt to share these design notes, which means the they will be uploaded when you FTP the files to the remote site, so that others that you work with when they download the file (and assuming they also have Design notes enabled and shared), then they will be able to see that a file has a design note associated with it. Note: These *.mno files are plain text and if you your server may be accessible by others, so be sure not to put anything confidential in them.
In the Files panel, right-click on a file that you want to add a TODO item to, select "Design Notes...". In the interface that opens, you can enter in a free form note for the document. Check the Show when file is opened to cause this interface to open when you open up the file you're applying the design note to.
If you switch to the All Info tab, you can add "keyed" notes, that is a note with a name, and an associated value. This open may be good for TODO items.
When you add a design note to a file and you have the Notes column displayed, you should see an icon in the Notes column for any files that have associated notes for. Double clicking this icon will open up the Design Notes UI. This will be a quick way to determine if you have any TODOs in the files that you can see. Note: you won't see this icon for files within folders that are closed.
If you would like to do some searches or listing of files with design notes associated with them, then you can take a look at the Site Reports Design Notes report. Go to Window -> Results -> Site Reports. In the upper right of this panel, click the green arrow. This brings up the Reports interface. Select the appropriate option under Report on, and then check "Design Notes" under Workflow. Don't bother with the report settings....it's supposed to allow for some search/filtering, but I can't get it not to display all files with notes associated with them, perhaps you'll have better luck. Click the Run button and the Site Reports panel should contain a list of files that have design notes associated with them, as well as the design note info for the file. If you double click the entry, then the file will open.