Update: doesn't seem to be specific to KMS, as I think I've been having the same problem with KMS disabled. Also, here are some messages I've been getting, though I'm not sure if they're relevant:
May 30 15:04:48 pia kernel: [ 36.520173] integrated sync not supported
May 30 15:04:48 pia kernel: [ 36.804170] integrated sync not supported
May 30 15:04:48 pia kernel: [ 37.080175] integrated sync not supported
May 30 15:04:52 pia kernel: [ 41.136237] integrated sync not supported
May 30 15:05:26 pia kernel: [ 74.736338] gvfs-gdu-volume[4741]: segfault at c ip 0065ffda sp bfdf6c10 error 4 in libgdu.so.0.0.0[657000+22000]
May 30 15:05:38 pia kernel: [ 87.120151] gvfs-gdu-volume[5034]: segfault at c ip 0050bfda sp bfe725b0 error 4 in libgdu.so.0.0.0[503000+22000]
May 30 15:05:44 pia kernel: [ 92.772304] mixer_applet2[4058]: segfault at aaaaaaac ip 00230be2 sp bfaf1a90 error 4 in libglib-2.0.so.0.2100.0[20d000+c3000]
May 30 15:05:46 pia kernel: [ 94.956223] integrated sync not supported
May 30 15:05:46 pia kernel: [ 95.240227] integrated sync not supported
May 30 15:05:48 pia kernel: [ 96.452239] integrated sync not supported
May 30 15:05:51 pia pulseaudio[5334]: pid.c: Stale PID file, overwriting.
May 30 15:05:53 pia kernel: [ 101.876205] integrated sync not supported
May 30 15:05:53 pia kernel: [ 102.156170] integrated sync not supported
May 30 15:05:54 pia kernel: [ 102.436207] integrated sync not supported
May 30 15:05:57 pia kernel: [ 105.448172] integrated sync not supported
May 30 15:06:14 pia kernel: [ 123.376261] integrated sync not supported
May 30 15:06:15 pia kernel: [ 123.652237] integrated sync not supported
May 30 15:06:16 pia kernel: [ 124.836227] integrated sync not supported
May 30 15:06:20 pia pulseaudio[5981]: pid.c: Stale PID file, overwriting.
May 30 15:06:22 pia kernel: [ 130.436180] integrated sync not supported
May 30 15:06:22 pia kernel: [ 130.716174] integrated sync not supported
May 30 15:06:22 pia kernel: [ 130.996176] integrated sync not supported
May 30 15:06:25 pia kernel: [ 134.256363] integrated sync not supported
May 30 15:06:30 pia bonobo-activation-server (jon-6227): could not associate with desktop session: Failed to connect to socket /tmp/dbus-A933wHNT6o: Connection refused
May 30 15:07:17 pia kernel: [ 185.424992] gvfs-gdu-volume[7108]: segfault at c ip 006ccfda sp bfe12650 error 4 in libgdu.so.0.0.0[6c4000+22000]
May 30 15:07:23 pia kernel: [ 192.085630] mixer_applet2[6171]: segfault at aaaaaaac ip 0039dbe2 sp bf87d150 error 4 in libglib-2.0.so.0.2100.0[37a000+c3000]
May 30 15:07:25 pia kernel: [ 194.144226] integrated sync not supported
May 30 15:07:26 pia kernel: [ 194.420241] integrated sync not supported
May 30 15:07:27 pia kernel: [ 195.604226] integrated sync not supported
May 30 15:07:31 pia pulseaudio[7427]: pid.c: Stale PID file, overwriting.
May 30 15:07:33 pia kernel: [ 201.572191] integrated sync not supported
May 30 15:07:33 pia kernel: [ 201.852172] integrated sync not supported
May 30 15:07:33 pia kernel: [ 202.136270] integrated sync not supported
May 30 15:07:36 pia kernel: [ 205.316192] integrated sync not supported
May 30 15:07:57 pia kernel: [ 226.235963] gvfs-gdu-volume[7681]: segfault at c ip 00544fda sp bf903440 error 4 in libgdu.so.0.0.0[53c000+22000]
Update: doesn't seem to be specific to KMS, as I think I've been having the same problem with KMS disabled. Also, here are some messages I've been getting, though I'm not sure if they're relevant:
May 30 15:04:48 pia kernel: [ 36.520173] integrated sync not supported volume[ 4741]: segfault at c ip 0065ffda sp bfdf6c10 error 4 in libgdu. so.0.0. 0[657000+ 22000] volume[ 5034]: segfault at c ip 0050bfda sp bfe725b0 error 4 in libgdu. so.0.0. 0[503000+ 22000] 4058]: segfault at aaaaaaac ip 00230be2 sp bfaf1a90 error 4 in libglib- 2.0.so. 0.2100. 0[20d000+ c3000] activation- server (jon-6227): could not associate with desktop session: Failed to connect to socket /tmp/dbus- A933wHNT6o: Connection refused volume[ 7108]: segfault at c ip 006ccfda sp bfe12650 error 4 in libgdu. so.0.0. 0[6c4000+ 22000] 6171]: segfault at aaaaaaac ip 0039dbe2 sp bf87d150 error 4 in libglib- 2.0.so. 0.2100. 0[37a000+ c3000] volume[ 7681]: segfault at c ip 00544fda sp bf903440 error 4 in libgdu. so.0.0. 0[53c000+ 22000]
May 30 15:04:48 pia kernel: [ 36.804170] integrated sync not supported
May 30 15:04:48 pia kernel: [ 37.080175] integrated sync not supported
May 30 15:04:52 pia kernel: [ 41.136237] integrated sync not supported
May 30 15:05:26 pia kernel: [ 74.736338] gvfs-gdu-
May 30 15:05:38 pia kernel: [ 87.120151] gvfs-gdu-
May 30 15:05:44 pia kernel: [ 92.772304] mixer_applet2[
May 30 15:05:46 pia kernel: [ 94.956223] integrated sync not supported
May 30 15:05:46 pia kernel: [ 95.240227] integrated sync not supported
May 30 15:05:48 pia kernel: [ 96.452239] integrated sync not supported
May 30 15:05:51 pia pulseaudio[5334]: pid.c: Stale PID file, overwriting.
May 30 15:05:53 pia kernel: [ 101.876205] integrated sync not supported
May 30 15:05:53 pia kernel: [ 102.156170] integrated sync not supported
May 30 15:05:54 pia kernel: [ 102.436207] integrated sync not supported
May 30 15:05:57 pia kernel: [ 105.448172] integrated sync not supported
May 30 15:06:14 pia kernel: [ 123.376261] integrated sync not supported
May 30 15:06:15 pia kernel: [ 123.652237] integrated sync not supported
May 30 15:06:16 pia kernel: [ 124.836227] integrated sync not supported
May 30 15:06:20 pia pulseaudio[5981]: pid.c: Stale PID file, overwriting.
May 30 15:06:22 pia kernel: [ 130.436180] integrated sync not supported
May 30 15:06:22 pia kernel: [ 130.716174] integrated sync not supported
May 30 15:06:22 pia kernel: [ 130.996176] integrated sync not supported
May 30 15:06:25 pia kernel: [ 134.256363] integrated sync not supported
May 30 15:06:30 pia bonobo-
May 30 15:07:17 pia kernel: [ 185.424992] gvfs-gdu-
May 30 15:07:23 pia kernel: [ 192.085630] mixer_applet2[
May 30 15:07:25 pia kernel: [ 194.144226] integrated sync not supported
May 30 15:07:26 pia kernel: [ 194.420241] integrated sync not supported
May 30 15:07:27 pia kernel: [ 195.604226] integrated sync not supported
May 30 15:07:31 pia pulseaudio[7427]: pid.c: Stale PID file, overwriting.
May 30 15:07:33 pia kernel: [ 201.572191] integrated sync not supported
May 30 15:07:33 pia kernel: [ 201.852172] integrated sync not supported
May 30 15:07:33 pia kernel: [ 202.136270] integrated sync not supported
May 30 15:07:36 pia kernel: [ 205.316192] integrated sync not supported
May 30 15:07:57 pia kernel: [ 226.235963] gvfs-gdu-