Can someone email you the .app package, or are Mac App Store apps compiled to be Snow Leopard only?
In terms of Snow Leopard-only, that's not necessarily true. However, it will be Intel-only.
The version of TapeDeck I submitted to the App Store had to be stripped of the ppc binary in order to pass validation. However, the version on http://tapedeckapp.com (the demo) is ppc/intel and 10.5-friendly.