I've captured the debug output of the outlined problem: Expires tag is less than the date of request processing, but there is no expiration message like "http file cache expired: 4 1302149578 1302149592"
Debug output:

2011/04/07 18:42:15 [debug] 10373#0: *6 epoll add event: fd:16 op:3 ev:80000005
2011/04/07 18:42:15 [debug] 10373#0: *6 http script var: "request"
2011/04/07 18:42:15 [debug] 10373#0: *6 http script var: "?"
2011/04/07 18:42:15 [debug] 10373#0: *6 http script var: "param=value"
2011/04/07 18:42:15 [debug] 10373#0: *6 http cache key: "request"
2011/04/07 18:42:15 [debug] 10373#0: *6 add cleanup: 0925FDD4
2011/04/07 18:42:15 [debug] 10373#0: *6 http file cache exists: 0 e:1
2011/04/07 18:42:15 [debug] 10373#0: *6 cache file: "/var/cache/a/22/ad1686833f0e0b0411e5bb582061822a"
2011/04/07 18:42:15 [debug] 10373#0: *6 add cleanup: 0925FE1C
2011/04/07 18:42:15 [debug] 10373#0: *6 http file cache fd: 21
2011/04/07 18:42:15 [debug] 10373#0: *6 read: 21, 0925FE68, 361, 0
2011/04/07 18:42:15 [debug] 10373#0: *6 http upstream cache: 0
2011/04/07 18:42:15 [debug] 10373#0: *6 http proxy status 200 "200 OK"
2011/04/07 18:42:15 [debug] 10373#0: *6 http proxy header: "Server: nginx"
2011/04/07 18:42:15 [debug] 10373#0: *6 http proxy header: "Date: Thu, 07 Apr 2011 11:16:40 GMT"
2011/04/07 18:42:15 [debug] 10373#0: *6 http proxy header: "Content-Type: text/xml"
2011/04/07 18:42:15 [debug] 10373#0: *6 http proxy header: "Connection: close"
2011/04/07 18:42:15 [debug] 10373#0: *6 http proxy header: "X-Powered-By: PHP/5.3.3-fpm.1"
2011/04/07 18:42:15 [debug] 10373#0: *6 http proxy header: "Cache-Control: public, must-revalidate, max-age=1800"
2011/04/07 18:42:15 [debug] 10373#0: *6 http proxy header: "Expires: Thu, 07 Apr 2011 11:25:35 GMT"
2011/04/07 18:42:15 [debug] 10373#0: *6 http proxy header: "ETag: 26ee7051c360ee28d2a5697fb2b97a03"
2011/04/07 18:42:15 [debug] 10373#0: *6 http proxy header: "Content-Length: 2102"
2011/04/07 18:42:15 [debug] 10373#0: *6 http proxy header done
2011/04/07 18:42:15 [debug] 10373#0: *6 http file cache send: /var/cache/a/22/ad1686833f0e0b0411e5bb582061822a
2011/04/07 18:42:15 [debug] 10373#0: *6 posix_memalign: 092300C0:4096 @16
2011/04/07 18:42:15 [debug] 10373#0: *6 HTTP/1.1 200 OK
Server: nginx/0.8.54
Date: Thu, 07 Apr 2011 11:42:15 GMT
Content-Type: text/xml
Connection: keep-alive
X-Powered-By: PHP/5.3.3-fpm.1
Cache-Control: public, must-revalidate, max-age=1800
Expires: Thu, 07 Apr 2011 11:25:35 GMT
ETag: 26ee7051c360ee28d2a5697fb2b97a03
Content-Length: 2102