The following was tested with the 20091012 snapshot in linux-backports-modules-alsa-2.6.31-14-generic (2.6.31-14.16):

testing hp_detect hint (unrelated due to misrouting)
testing different init verbs due to 5stack and 5stack-no-fp models

The key here is that vref and routing have regressed due to the autodetect changes in 2.6.29+:

$ diff -uN codec_hp_inserted_after_powercycle_2.6.28-13-generic.txt codec_hp_inserted_after_powercycle.txt
--- codec_hp_inserted_after_powercycle_2.6.28-13-generic.txt	2009-10-15 16:07:41.000000000 -0400
+++ codec_hp_inserted_after_powercycle.txt	2009-10-15 15:11:27.000000000 -0400
@@ -1,5 +1,6 @@
 Codec: SigmaTel STAC9227
 Address: 2
+Function Id: 0x1
 Vendor Id: 0x83847618
 Subsystem Id: 0x80862008
 Revision Id: 0x100201
@@ -11,30 +12,31 @@
 Default Amp-In caps: ofs=0x00, nsteps=0x0e, stepsize=0x05, mute=0
 Default Amp-Out caps: ofs=0x7f, nsteps=0x7f, stepsize=0x02, mute=1
 GPIO: io=3, o=0, i=0, unsolicited=1, wake=1
-  IO[0]: enable=1, dir=1, wake=0, sticky=0, data=1
-  IO[1]: enable=0, dir=0, wake=0, sticky=0, data=0
-  IO[2]: enable=0, dir=0, wake=0, sticky=0, data=0
+  IO[0]: enable=1, dir=1, wake=0, sticky=0, data=1, unsol=0
+  IO[1]: enable=0, dir=0, wake=0, sticky=0, data=0, unsol=0
+  IO[2]: enable=0, dir=0, wake=0, sticky=0, data=0, unsol=0
+Analog Loopback: 0x00
 Node 0x02 [Audio Output] wcaps 0xd0c05: Stereo Amp-Out R/L
   Amp-Out caps: N/A
-  Amp-Out vals:  [0xff 0xff]
+  Amp-Out vals:  [0x80 0x80]
   Converter: stream=0, channel=0
   Power: setting=D0, actual=D0
   Delay: 13 samples
 Node 0x03 [Audio Output] wcaps 0xd0c05: Stereo Amp-Out R/L
   Amp-Out caps: N/A
-  Amp-Out vals:  [0xff 0xff]
+  Amp-Out vals:  [0x6b 0x6b]
   Converter: stream=0, channel=0
-  Power: setting=D3, actual=D3
+  Power: setting=D0, actual=D0
   Delay: 13 samples
 Node 0x04 [Audio Output] wcaps 0xd0c05: Stereo Amp-Out R/L
   Amp-Out caps: N/A
-  Amp-Out vals:  [0xff 0xff]
+  Amp-Out vals:  [0x80 0x80]
   Converter: stream=0, channel=0
   Power: setting=D0, actual=D0
   Delay: 13 samples
 Node 0x05 [Audio Output] wcaps 0xd0c05: Stereo Amp-Out R/L
   Amp-Out caps: N/A
-  Amp-Out vals:  [0xff 0xff]
+  Amp-Out vals:  [0x80 0x80]
   Converter: stream=0, channel=0
   Power: setting=D0, actual=D0
   Delay: 13 samples
@@ -73,10 +75,10 @@
   Pin Default 0x0221401f: [Jack] HP Out at Ext Front
     Conn = 1/8, Color = Green
     DefAssociation = 0x1, Sequence = 0xf
-  Pin-ctls: 0xc0: OUT HP VREF_HIZ
-  Unsolicited: tag=30, enabled=1
+  Pin-ctls: 0x00: VREF_HIZ
+  Unsolicited: tag=01, enabled=1
   Connection: 2
-     0x02* 0x03
+     0x02 0x03*
 Node 0x0b [Pin Complex] wcaps 0x400181: Stereo
   Pincap 0x0000173f: IN OUT HP Detect Trigger ImpSense
     Vref caps: HIZ 50 GRD 80
@@ -85,7 +87,7 @@
     DefAssociation = 0x2, Sequence = 0x0
     Misc = NO_PRESENCE
   Pin-ctls: 0x24: IN VREF_80
-  Unsolicited: tag=00, enabled=0
+  Unsolicited: tag=03, enabled=1
   Connection: 2
      0x02 0x03*
 Node 0x0c [Pin Complex] wcaps 0x400181: Stereo
@@ -116,7 +118,7 @@
     Conn = 1/8, Color = Pink
     DefAssociation = 0x2, Sequence = 0x1
   Pin-ctls: 0x24: IN VREF_80
-  Unsolicited: tag=00, enabled=0
+  Unsolicited: tag=02, enabled=1
   Connection: 1
      0x04
 Node 0x0f [Pin Complex] wcaps 0x400181: Stereo
@@ -125,8 +127,8 @@
   Pin Default 0x01813024: [Jack] Line In at Ext Rear
     Conn = 1/8, Color = Blue
     DefAssociation = 0x2, Sequence = 0x4
-  Pin-ctls: 0x60: IN OUT VREF_HIZ
-  Unsolicited: tag=00, enabled=0
+  Pin-ctls: 0x20: IN VREF_HIZ
+  Unsolicited: tag=04, enabled=1
   Connection: 1
      0x05
 Node 0x10 [Pin Complex] wcaps 0x400181: Stereo
@@ -175,7 +177,7 @@
      0x0e* 0x12 0x0f 0x0b 0x0c 0x0d 0x0a 0x10 0x11
 Node 0x18 [Audio Selector] wcaps 0x300103: Stereo Amp-In
   Amp-In caps: N/A
-  Amp-In vals:  [0x00 0x00]
+  Amp-In vals:  [0x08 0x08]
   Connection: 1
      0x15
 Node 0x19 [Audio Selector] wcaps 0x300103: Stereo Amp-In
@@ -252,4 +254,4 @@
 Node 0x24 [Volume Knob Widget] wcaps 0x600000: Mono
   Volume-Knob: delta=1, steps=127, direct=1, val=127
   Connection: 4
-     0x02* 0x03 0x04 0x05
+     0x02 0x03 0x04 0x05