One challenge in Access Services is ensuring that the logic behind various objects will be portable between Access itself and the SharePoint site hosting Access web database. To that end, Access team has used macros to support the goal of portability. In practical terms, this means we cann't use VBA behind Access objects in context of a web browser.
Richard Fennell, has written about his experiences with Access 2010, that we can use SharePoint object model and Visual Studio 2010 to introduce business logic to apps. With access to .NET framework and a full-bodied language, web databases needn't be limited by the macros. Here's a proof of concept demonstration: