Reproduced this under osx 10.5.2 (intel and ppc).
zip archive size is about 7.5 MB
canceling the download is fine; canceling the extraction causes the crash.
Thread 12 Crashed:
0 libobjc.A.dylib 0x90017878 objc_msgSend_fpret + 24
1 org.andymatuschak.Sparkle 0x000af14c -[SUUnarchiver extractArchivePath:pipingDataToCommand:] + 504
2 org.andymatuschak.Sparkle 0x000af2a8 -[SUUnarchiver _extractZIP:] + 50
3 com.apple.CoreFoundation 0x93be0b7d __invoking___ + 29
4 com.apple.CoreFoundation 0x93be0568 -[NSInvocation invoke] + 136
5 com.apple.CoreFoundation 0x93be0638 -[NSInvocation invokeWithTarget:] + 72
6 org.andymatuschak.Sparkle 0x000af5e3 -[SUUnarchiver _unarchivePath:] + 389
7 com.apple.Foundation 0x968095ad -[NSThread main] + 45
8 com.apple.Foundation 0x96809154 __NSThread__main__ + 308
9 libSystem.B.dylib 0x93dc8c55 _pthread_start + 321
10 libSystem.B.dylib 0x93dc8b12 thread_start + 34