No need to buy a new compatible receiver. Just buy a $20 splitter and that should get the job done. You could put off that upgrade a few more years.
On the published specs for the Apple TV for "Audio Formats" it lists:
HE-AAC (V1), AAC (16 to 320 Kbps), protected AAC (from iTunes Store), MP3 (16 to 320 Kbps), MP3 VBR, Audible (formats 2, 3, and 4), Apple Lossless, AIFF, and WAV; Dolby Digital 5.1 and Dolby Digital Plus 7.1
So I guess we're assuming that the Apple TV will have a setting that will let you switch the audio output to either Dolby Digital 5.1 or Dolby Digital Plus 7.1? From what I understand those cheap splitters can't send Dolby Digital Plus over optical audio out.