Re-ran qa-regression-tests (in a schroot) with the package from -proposed:

(natty32)gimre@voy:~/WORK$ sudo qa-regression-testing/scripts/test-postfix.py -v
Running test: 'qa-regression-testing/scripts/test-postfix.py' distro: 'Ubuntu 11.04' kernel: 'found' arch: 'i386' uid: 0/0 SUDO_USER: 'gimre')
test_00_listening (__main__.PostfixTest)
Postfix is listening ... ok
test_10_commands (__main__.PostfixTest)
Basic SMTP commands ... ok
test_10_sending_mail_direct (__main__.PostfixTest)
Mail delivered normally ... ok
test_10_sending_mail_direct_auth (__main__.PostfixTest)
Mail authentication ... ok
test_10_sending_mail_direct_auth_full (__main__.PostfixTest)
Mail delivered with authentication ... ok
test_10_sending_mail_direct_with_tls (__main__.PostfixTest)
Mail delivered normally with TLS ... ok
test_10_sending_mail_forward_normal (__main__.PostfixTest)
Mail delivered via .forward ... ok
test_10_sending_mail_forward_xternal (__main__.PostfixTest)
Mail processed by commands in .forward ... ok
test_11_security_CVE_2008_2936 (__main__.PostfixTest)
CVE-2008-2936 fixed ... ok
test_20_sasldb_cram_md5 (__main__.PostfixTest)
Test sasldb CRAM-MD5 ... ok
test_20_sasldb_digest_md5 (__main__.PostfixTest)
Test sasldb DIGEST-MD5 is supported ... ok
test_20_sasldb_login (__main__.PostfixTest)
Test sasldb LOGIN is supported ... ok
test_20_sasldb_plain (__main__.PostfixTest)
Test sasldb PLAIN ... ok
test_21_security_CVE_2011_1720 (__main__.PostfixTest)
CVE-2011-1720 fixed ... ok
test_99_restore (__main__.PostfixTest)
Restore configuration ... ok

----------------------------------------------------------------------
Ran 15 tests in 66.698s

OK

(natty32)gimre@voy:~/WORK$ apt-cache policy postfix
postfix:
  Installed: 2.8.5-2~build0.11.04
  Candidate: 2.8.5-2~build0.11.04
  Version table:
 *** 2.8.5-2~build0.11.04 0
        500 http://voy/ubuntu/ natty-proposed/main i386 Packages
        100 /var/lib/dpkg/status
     2.8.2-1ubuntu1 0
        500 http://voy/ubuntu/ natty/main i386 Packages

Installed the package in a VM, with default 'Internet site' configuration, it installed without problems. Sent email through it, email got delivered to local mailbox (/var/spool/mail/<user>).

Removed package, installed the last one from updates/security (2.8.2-1ubuntu2.1), then did an update to 2.8.5-2~build0.11.04 from -proposed, seems to update OK. Not sure why it's stopping twice, but I guess that's normal.

Preparing to replace postfix 2.8.2-1ubuntu2.1 (using .../postfix_2.8.5-2~build0.11.04_i386.deb) ...
 * Stopping Postfix Mail Transport Agent postfix                                                                                                                                                                                                                                [ OK ] 
 * Stopping Postfix Mail Transport Agent postfix                                                                                                                                                                                                                                [ OK ] 
Unpacking replacement postfix ...
[...]
Setting up postfix (2.8.5-2~build0.11.04) ...

Postfix configuration was not changed.  If you need to make changes, edit
/etc/postfix/main.cf (and others) as needed.  To view Postfix configuration
values, see postconf(1).

After modifying main.cf, be sure to run '/etc/init.d/postfix reload'.

Running newaliases
 * Stopping Postfix Mail Transport Agent postfix                                                                                                                                                                                                                                [ OK ] 
 * Starting Postfix Mail Transport Agent postfix                                                                                                                                                                                                                                [ OK ]