This was reported in IRC and I don't have a test case, but creating one should
be simple:

00:33 <bringert> darcs 2.0.2 overflows the stack when running 'darcs what' in a 
                 repo where a small file has been replaced by a 3M text file 
                 containing no newlines
00:34 <twb> bringert: did you build with GHC 6.8.3 or .2?
00:34 <bringert> 6.8.2
00:34 <twb> There are known issues with 6.8.2
00:36 ~~~ bringert upgrades ghc
<snip>
02:47 <bringert> darcs 2.0.2 compiled with GHC 6.8.3 has the same stack 
                 overflow problem

My hunch is that is most likely lies in smart_diff or maybe some code that
creates a patch to display the results of whatsnew.

Steps to reproduce:

$ darcs init 
$ ghc -e "writeFile \"foo\" (replicate 4000 'x')"
$ darcs add foo
$ darcs record -a -m 'foo' foo
$ ghc -e "writeFile \"foo\" (replicate 4000000 'y')"
$ darcs what
Stack space overflow: current size 8000016 bytes.
Use `+RTS -Ksize' to increase it.

This is now fixed on the sprint branch (http://code.haskell.org/darcs/sprint/)
by either of the following two patches. They both make sense anyway, so I will
submit them both.

Sat Oct 25 13:23:31 BST 2008  Ganesh Sittampalam <ganesh@earth.li>
  * just hash the last 20 characters in LCS

Sat Oct 25 13:18:57 BST 2008  Ganesh Sittampalam <ganesh@earth.li>
  * accumulate hash strictly in hash/hashPS

This is fixed in darcs 2.2.0. Only the "accumulate hash strictly" patch was kept
as the "hash last 20 characters" was actually a bad idea.