Getting OpenSSL to handle md5 didn't change anything (wasn't able to get it to compile with your specific flag, but I figured this was what you had intended?)

Here's a sample from the debug log:

2012/09/13 15:15:16 [debug] 27367#0: *3441 http cache key: "http://origin_magick"
2012/09/13 15:15:16 [debug] 27367#0: *3441 http cache key: "/uploads/channel/thumbnail/5927/e74e97624ef652bc7a8419c02f8a7f92ae71d127.jpg"
2012/09/13 15:15:16 [debug] 27367#0: *3441 add cleanup: 000000000196CE68
2012/09/13 15:15:16 [debug] 27367#0: *3441 http file cache exists: 0 e:1
2012/09/13 15:15:16 [debug] 27367#0: *3441 cache file: "/mnt/tmp/nginx/cache/90/6d/2b/422c25423170bac1c4e655ed9c2b6d90"
2012/09/13 15:15:16 [debug] 27367#0: *3441 add cleanup: 000000000196CEC0
2012/09/13 15:15:16 [debug] 27367#0: *3441 http file cache fd: 24
2012/09/13 15:15:16 [debug] 27367#0: *3441 read: 24, 000000000196D198, 285, 0
2012/09/13 15:15:16 [crit] 27367#0: *3441 cache file "/mnt/tmp/nginx/cache/90/6d/2b/422c25423170bac1c4e655ed9c2b6d90" has md5 collision, client: 127.0.0.1, server: atlas-x.example.com, request: "GET /uploads/channel/thumbnail/5927/e74e97624ef652bc7a8419c02f8a7f92ae71d127.jpg HTTP/1.1", host: "127.0.0.1"
2012/09/13 15:15:16 [debug] 27367#0: *3441 http upstream cache: -5