Normally, when I record a patch, darcs prompts me to edit the long comment. I
did the following:

$ darcs record -m "Lightweight ticky-ticky profiling" ./compiler/cmm/CLabel.hs
./compiler/codeGen/CgClosure.lhs ./compiler/codeGen/CgTicky.hs
./compiler/codeGen/SMRep.lhs ./compiler/main/CodeOutput.lhs
./compiler/main/StaticFlags.hs ./includes/Cmm.h ./includes/README
./includes/Rts.h ./includes/StgTicky.h ./includes/TickyCounters.h
./includes/mkDerivedConstants.c ./rts/RtsStartup.c ./rts/StgMiscClosures.cmm
./rts/Ticky.c ./rts/Updates.cmm ./rts/Updates.h ./utils/genapply/GenApply.hs 
Recording changes in "compiler/cmm/CLabel.hs" "compiler/codeGen/CgClosure.lhs"
"compiler/codeGen/CgTicky.hs" "compiler/codeGen/SMRep.lhs"
"compiler/main/CodeOutput.lhs" "compiler/main/StaticFlags.hs" "includes/Cmm.h"
"includes/README" "includes/Rts.h" "includes/StgTicky.h"
"includes/TickyCounters.h" "includes/mkDerivedConstants.c" "rts/RtsStartup.c"
"rts/StgMiscClosures.cmm" "rts/Ticky.c" "rts/Updates.cmm" "rts/Updates.h"
"utils/genapply/GenApply.hs":

hunk ./includes/StgTicky.h 1
[snip]
Shall I record this change? (1/?)  [ynWsfqadjkc], or ? for help: a

Finished recording patch 'Lightweight ticky-ticky profiling'
krc@bongo:~/ghc-hacking$ 

I assume it must be because I supplied a file list that I didn't get prompted to
edit the long comment, and when I unrecorded the patch and re-recorded it with
--edit-long-comment, it worked, but this seems like a bug.

No.  When you pass a patch name via -m, you don't get the option to put
in a long comment.  I think this behaviour is ok as-is.

That said, there is a --prompt-long-comment switch, but this also gets
ignored with -m.  I think it would be fair to ask that
--prompt-long-comment with -m give the behaviour you're expecting.

Resolved in unstable with:

Fri Jul 27 22:35:03 CEST 2007  Eric Kow <eric.kow@loria.fr>
  * [issue389] Support record -m --prompt-long-comment.