Why is the Pressflow Last-Modified header not like plain Drupal?
Drupal 6 sets the Last-Modified header with

header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");


That was changed to

$default_headers = array(
  'Expires' =&gt; 'Sun, 19 Nov 1978 05:00:00 GMT',
  'Last-Modified' =&gt; gmdate(DATE_RFC1123, REQUEST_TIME),


during the development of drupal7. Pressflow has backported the patch from drupal7 in 2009. That's the reason of the difference. As for the possible impact of this, in fact it can have bad effects on caching. At least squid 3.1.10 prints a warning about the header and refuses to store the page in cache

cannot parse hdr field: 'Last-Modified: Tue, 12 Feb 2013 14:35:43 +0000'
[snip]...[/snip]
refreshIsCachable() returned non-cacheable..


Switching to the "GMT" version resolves the issue with squid.

rfc2616 (HTTP/1.1: Protocol Parameters:) specifies that "Last-Modified" header value can conform to rfc1123 (which looks OK with +0000) but all the examples use "GMT".

I think it'll be better to create a bug report in drupal issue tracker and see what the developers answer.