It is possible to modify the 300D (as well as most other DSLRs) to greatly enhance sensitivity to astronomical objects (see spectral response tests and pre mod/post mod examples). This involves removal of the standard IR cut/AA filter. It is not that hard to do but does require some previous experience working with small electronics (estimated modification time ~2-20 hours). I have a mini-procedure for doing this mod here. Even with these skills there is a risk that the camera may be damaged/destroyed during the modifcation.

Christian Buil's Modifying Canon 350D/XT.

Gary Honis excellent 300D/Rebel IRC removal procedure.

Canon Introduces the EOS 20Da for astrophotography!

Excellent 10D IRC removal procedure by Manfred Schwarz.

Maxmax sell an external mount filter that matches the standard Canon filter - the XNite CC1 is the one required. New! Maxmax also modify the 300D.

New! Achim Schaller's procedure for modifying the Canon 10D.

The first inspiration for this modification came from this site http://www.seo-e.co.jp/hobby.htm

Other links of interest include:

Canon 10D Mod (Japanese)

Hutech sell pre-modified 300D's and 20Ds

New! Canon 20D Mod (Tim Cann)

Nikon D70 Mod (Christian Buil)

Canon's own Astrophotography Page

Lastly doing this modification has some major implications for daylight use, please refer to this link for details.