In relation to ODBC data sources, what the Enterprise License gives you in Excel Services is the ability to externalize your data connections so they may be reused across workbooks. With the Standard License, you do not get Excel Calculation Services, so what you end up with essentially is Excel files stored in a document library. To that end, you may do anything with ODBC data sources that you can do from your workstation, because you must open the Excel doc on your workstation to use it -- SharePoint is simply serving as a repository.