Found a nasty bug in rtmpdump. The buffer that is used internally to construct the tcUrl, if it isn't explicitily passed, is too small. Here's an example from the BBC. First what I passed rtmpdump and then you can see the information it echoed back. That's not just a truncated display of the tcUrl, that's actually what it is passing the server. The work-around is to explicitly pass the tcUrl, and everything is dandy.
rtmpdump
-a "a1414/e3?as=adobe-hmac-sha256&av=1&te=connect&mp=iplayerstream/secure_auth/480kbps/MP/b00tyr8w_1320405462.mp4,iplayerstream/secure_auth/480kbps/MP/b00tyr8w_1320405462.mp4,iplayerstream/secure_auth/1500kbps/MP/b00tyr8w_1320405154.mp4,iplayerstream/secure_auth/400kbps/b00tyr8w_1320405462.mp4,iplayerstream/secure_auth/800kbps/MP/b00tyr8w_1320405470.mp4,iplayerstream/secure_auth/1500kbps/MP/b00tyr8w_1320405154.mp4,iplayerstream/secure_auth/800kbps/MP/b00tyr8w_1320405470.mp4,iplayerstream/secure_auth/100kbps/b00tyr8w_1320405462.mp4,prod_3gp_mpeg4_med/iplayerstream/prod_3gp_mpeg4_med/wifi_CBeebies_Bedtime_Stories_Episode_179_b00tyr8w _1320405462.mp4&et=1320450031&fmta-token=22853c1460014856485cfba5469a8b4014e6434ef464 a015f6fb0ef6d6ec4432" --host "bbcmedia.fcod.llnwd.net" --playpath "mp4:iplayerstream/secure_auth/1500kbps/MP/b00tyr8w_1320405154.mp4" -s "http://www.bbc.co.uk/emp/iplayer/revisions/617463_618125_3/617463_618125_3_emp.swf" --port 1935 --protocol "rtmp" -o activedl.flv
RTMPDump v2.3
(c) 2010 Andrej Stepanchuk, Howard Chu, The Flvstreamer Team; license: GPL
DEBUG: Protocol : RTMP
DEBUG: Hostname : bbcmedia.fcod.llnwd.net
DEBUG: Port : 1935
DEBUG: Playpath : mp4:iplayerstream/secure_auth/1500kbps/MP/b00tyr8w_1320405154.mp4
DEBUG: tcUrl : rtmp://bbcmedia.fcod.llnwd.net:1935/a1414/e3?as=adobe-hmac-sha256&av=1&te=connect&mp=iplayerstream/secure_auth/480kbps/MP/b00tyr8w_1320405462.mp4,prod_3gp_mpeg4_med/iplayerstream/prod_3gp_mpeg4_med/wifi_CBeebies_Bedtime_Stories_Episode_179_b00tyr8w _1320405462.mp4,iplayerstream/secure_auth/1500kbps/MP/b00tyr8w_1320405154.mp4,iplayerstream/secure_auth/400kbps/b00tyr8w_1320405462.mp4,iplayerstream/secure_auth/800kbps/MP/b00tyr8w_1320405470.mp4,iplayerstream/secure_auth/100kbps/b00tyr8w_1320405462.mp4,iplayer
DEBUG: swfUrl :
http://www.bbc.co.uk/emp/iplayer/rev...8125_3_emp.swf
DEBUG: pageUrl :
http://www.bbc.co.uk/iplayer/episode..._Perfect_Baby/
DEBUG: app : a1414/e3?as=adobe-hmac-sha256&av=1&te=connect&mp=iplayerstream/secure_auth/480kbps/MP/b00tyr8w_1320405462.mp4,prod_3gp_mpeg4_med/iplayerstream/prod_3gp_mpeg4_med/wifi_CBeebies_Bedtime_Stories_Episode_179_b00tyr8w _1320405462.mp4,iplayerstream/secure_auth/1500kbps/MP/b00tyr8w_1320405154.mp4,iplayerstream/secure_auth/400kbps/b00tyr8w_1320405462.mp4,iplayerstream/secure_auth/800kbps/MP/b00tyr8w_1320405470.mp4,iplayerstream/secure_auth/100kbps/b00tyr8w_1320405462.mp4,iplayerstream/secure_auth/480kbps/MP/b00tyr8w_1320405462.mp4,iplayerstream/secure_auth/1500kbps/MP/b00tyr8w_1320405154.mp4,iplayerstream/secure_auth/800kbps/MP/b00tyr8w_1320405470.mp4&et=1320449266&fmta-token=cfbab6649ad53ebf96a89308615c66c7849520b86a9a c12c2c25d49dcfcad134
DEBUG: live : yes
DEBUG: timeout : 30 sec
DEBUG: Setting buffer time to: 36000000ms
Connecting ...
DEBUG: RTMP_Connect1, ... connected, handshaking
DEBUG: HandShake: Type Answer : 03
DEBUG: HandShake: Server Uptime : 189404762
DEBUG: HandShake: FMS Version : 4.0.3.1
DEBUG: HandShake: Handshaking finished....
DEBUG: RTMP_Connect1, handshaked