Hmmm .... this problem recurred after the reinstall, but it's only happened once in five boots. I'm going to gather as much info as I can, keep the bug report open, and add whatever useful info I can to the report.
The road I'm going to go down for now is that this may be the result of an interaction between gnome and the nouveau driver. Before the offending reboot messages of the following sort were spewed into the messages file:
May 25 19:15:50 nerowolfe kernel: [29293.093583] [drm] nouveau 0000:00:0d.0: PGRAPH - ch 3 (0x0006b000) subc 4 class 0x0039 mthd 0x0300 data 0x00000003
May 25 19:15:50 nerowolfe kernel: [29293.093588] [drm] nouveau 0000:00:0d.0: PGRAPH - ERROR nsource: ILLEGAL_MTHD nstatus: BAD_ARGUMENT PROTECTION_FAULT
May 25 19:15:50 nerowolfe kernel: [29293.093594] [drm] nouveau 0000:00:0d.0: PGRAPH - ch 3 (0x0006b000) subc 4 class 0x0039 mthd 0x0304 data 0x00000003
May 25 19:15:50 nerowolfe kernel: [29293.093598] [drm] nouveau 0000:00:0d.0: PGRAPH - ERROR nsource: ILLEGAL_MTHD nstatus: BAD_ARGUMENT PROTECTION_FAULT
May 25 19:15:50 nerowolfe kernel: [29293.093604] [drm] nouveau 0000:00:0d.0: PGRAPH - ch 3 (0x0006b000) subc 4 class 0x0039 mthd 0x0308 data 0x012001df
May 25 20:02:16 nerowolfe kernel: [32079.928230] nouveau_ratelimit: 32 callbacks suppressed
May 25 20:02:16 nerowolfe kernel: [32079.928238] [drm] nouveau 0000:00:0d.0: PGRAPH - ERROR nsource: DATA_ERROR nstatus: BAD_ARGUMENT PROTECTION_FAULT
May 25 20:02:16 nerowolfe kernel: [32079.928257] [drm] nouveau 0000:00:0d.0: PGRAPH - ch 3 (0x0006b000) subc 2 class 0x3089 mthd 0x0300 data 0x0a0a000a
Further, in the exchange in the first fedora bug report in Brian's links earlier in this thread a similar problem was diagnosed in terms of the nouveau driver. At this point I'm going to pull together the diagnostic info suggested in the fedora bug report.