When you press Alt+Tab, it, indeed, shows all the applications (on all workspaces), but separates the applications on different workspaces by vertical lines. So, while this is not exactly what you want, it still may help a little.

I would suggest you install Gnome-Shell extension Window List (it is written for Linux Mint 12 Lisa and now available for Ubuntu in the WebUpd8 Gnome3 PPA). This extension shows buttons on the panel (on the top panel if the bottom panel, another their extension, is not installed or disabled) for all applications open in the current workspace only (open and minimized). They have application icons and title.

These buttons work as expected: when you click an unfocused application button, it brings it on top, when you click a button for the currently focused application, it minimizes it. When you switch the workspace, it shows the buttons for the applications on the new workspace.

This way you will have "Alt-Tab" list of applications on the current workspace always visible and at your hands. The only problem is that there is no shortcut to switch between them, you have to use mouse.

This is, probably, the best I could suggest for you at the moment.