Can I still use my dash controls with an aftermarket head unit?
Maybe. Without some sort of interface? no.
However, you can reposess the controls and add features to the controls by either adding the DCI from modifry or having DJQube (an s2ki.com user) modify it. Links to both of these can be found below in the section on shopping online. These are the most commonly used and recommended solutions.

The PAC SWI-X is also an inexpensive option. It can be bought from Crutchfield or locally with a little bit of looking. The major flaw with the PAC SWI-X is that it must be programmed for your car and HU. The nature of this programming creates a slight, but noticable delay when using the dash controls.

In order to use ANY of these options above, the Head Unit you purchase MUST have an InfraRed remote. Some Sony models are an exception to this rule, as they can use a hardwired option.

Some HU's in Alpine's new 2005 line also have a hardwire line input for remote dash controls. As of this writing (7/2005) the interface for the S2000 is mainly available in Europe, but has been sighted as in the US. Some users have imported the interface to the US from Europe with good success.

PAC also offers a programmable option for the dash controls that hardwires into the 2005 Alpine head units. It is called the SWI-ALP and is currently (as of 6/2005) available in the US. It requires an Alpine HU w/ the hardwire input, so double check before you buy.

Unlike modifry and DJQube's DCI's, the PAC and Alpine options do not add features, but instead mimic the S2000's OEM controls.