My setting:

(setq nnmail-spool-file `((directory :path
				     ,(expand-file-name "~/.Mail/incoming"))
			  (file :path "/usr/spool/mail/dsg")))

I just upgraded from 0.77 to 0.79 and my nnml mail no longer gets
incorporated.  It turns out to be the default predicate for
nnmail-spool-file.  It's setting the string to be matched to
<groupname>$ instead of just <groupname>.  Since I use the default
suffix (".spool"), the filenames always fail the string-match as a
result.  Here's the relevant spot in a backtrace which I got by
setting debug-on-entry and did M-g over nnml:inbox (lines wrapped and
spaced by me for readability):

Return value: nil
  string-match("inbox$" "/afs/rcf/user/dsg/.Mail/incoming/inbox.spool")

* (lambda (file) (string-match "inbox$" file))
  ("/afs/rcf/user/dsg/.Mail/incoming/inbox.spool")

* mail-source-fetch-directory((directory
			       :path "/afs/rcf/user/dsg/.Mail/incoming"
			       :predicate
			       (lambda (file) (string-match "inbox$" file)))
			      (lambda (file orig-file)
				(nnmail-split-incoming
				 file (quote nnml-save-mail) (quote nil)
				 (nnmail-get-split-group orig-file source)
				 (quote nnml-active-number))))

* mail-source-fetch((directory
		     :path "/afs/rcf/user/dsg/.Mail/incoming"
		     :predicate
		     (lambda (file) (string-match "inbox$" file)))
		    (lambda (file orig-file)
		      (nnmail-split-incoming
		       file (quote nnml-save-mail) (quote nil)
		       (nnmail-get-split-group orig-file source)
		       (quote nnml-active-number))))

* nnmail-get-new-mail(nnml nnml-save-nov "/afs/rcf/user/dsg/.Mail" "inbox")
* nnml-request-scan("inbox" "")
* gnus-request-scan("nnml:inbox" (nnml ""))
* gnus-activate-group("nnml:inbox" scan)
* gnus-group-get-new-news-this-group(nil)
* byte-code("À ¬„Á\n!‡ÃÄ !ˆÁ ‡"
	    [gnus-group-topic-p gnus-group-get-new-news-this-group n
				gnus-topic-mark-topic gnus-group-topic-name] 2)
* gnus-topic-get-new-news-this-topic(nil)
  call-interactively(gnus-topic-get-new-news-this-topic)

I realize that since I use a separate directory for incoming files,
that the suffix is redundant.  However, the suffix ".spool" is still
the default (without setting it myself, I saw it being used in the
backtrace as I stepped through it) and as such it shouldn't have
broken anything.  It looks like the right fix would be to do a
mail-source-bind just before setting the predicate and incorporating
the suffix into it.  Attached is a patch.