Changes have been made in Git (master):

commit 88f7e2b8bcff65b0346cdb7d0f61de57d37487a6
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Mon Jan 30 00:26:05 2012 -0700

     Move AJAX mailbox response generation entirely within IMP_Imap_Tree

     Precursor to implementing Ticket #8077

  imp/js/dimpbase.js           |    2 +-
  imp/lib/Ajax/Application.php |   51 ++++++---------------
  imp/lib/Ajax/Queue.php       |    4 +-
  imp/lib/Imap/Tree.php        |  103 
++++++++++++++++++++++--------------------
  imp/lib/Mailbox.php          |    1 -
  5 files changed, 72 insertions(+), 89 deletions(-)

http://git.horde.org/horde-git/-/commit/88f7e2b8bcff65b0346cdb7d0f61de57d37487a6

any progress on this? thank you for info.

No, this ticket does not implement the previous functionality.  The 
old code was nothing more than a web interface to a (not very 
optimized) PHP port of fetchmail.  Which wasn't all that useful for 
anyone with more than a few hundred messages, since PHP would time-out 
or memory fail long before you could synchronize an account.

This ticket is instead a request to add foreign accounts to appear 
natively in the UI.  Which would allow access of these accounts 
without expensive overhead (and allow the synchronization of mail 
actions).
I would love to see this request also.  But the people that currently 
pay me to work on IMP do not have this as a priority.  Unfortunately, 
that is the reality of the situation, especially for something as 
resource intensive as this request.

Should've posted the English website:
http://phlymail.com/en/phlymail/lite/

In case it's of anybodies interest: I switched to PhlyMail which seems 
to be the only product offering a neat integration of multiple email 
accounts in one interface: http://phlymail.com/de/phlymail/lite/
(unfortunately without drag&drop and without a mobile client though)

This feature is discussed (in a positive, it just works, sense) a lot 
online because it has been a feature for a long time with IMP3/4. It 
was quite hard to to find this page and properly confirm that it is 
now depreciated in IMP5. I have added a (rather terse, sorry, I was 
suffering a sense of humour failure) note to the Wiki page about this.

From the search results seen before I found this page I imagine that 
hosting providers (plesk/cpanel based) will have a strong incentive to 
get this restored since they have been offering IMP as a mail 
aggregator service to their customers. They may well feel that this 
issue is a blocker for IMP5 adoption.

As per the user below I'd also been hoping to use IMP5 to aggregate 
some external mailboxes into my main mail system, and might well have 
stayed with OpenWebmail if I had seen in advance that I would loose 
this functonality. I have worked around it by running getmail as a 
service on my server, but it's harder to configure and maintain than 
if it was a native IMP function.

Changes have been made in Git (refs/heads/ajax_to_core):

commit 88f7e2b8bcff65b0346cdb7d0f61de57d37487a6
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Mon Jan 30 00:26:05 2012 -0700

     Move AJAX mailbox response generation entirely within IMP_Imap_Tree

     Precursor to implementing Ticket #8077

  imp/js/dimpbase.js           |    2 +-
  imp/lib/Ajax/Application.php |   51 ++++++---------------
  imp/lib/Ajax/Queue.php       |    4 +-
  imp/lib/Imap/Tree.php        |  103 
++++++++++++++++++++++--------------------
  imp/lib/Mailbox.php          |    1 -
  5 files changed, 72 insertions(+), 89 deletions(-)

http://git.horde.org/horde-git/-/commit/88f7e2b8bcff65b0346cdb7d0f61de57d37487a6

Would nice to have this function in IMP.
I donate some dollars to Horde LLC today.

I love Horde and honestly the ability to manage multiple imap accounts 
is the only missing feature for me.

Oh ... that's very sad to hear. :(
I really hope this might change in future...

Zero progress; no ETA.  Someone will probably need to fund this 
feature in order for it to get done.

Searching for the best web email client every second page pointed to 
Horde IMP. So I took the time to get it working (the installation 
process took me a couple of days...).

Now having everything set up I notice the for me most important 
feature: to have multiple IMAP accounts accessable at the same time; 
represented in the tree on the left, like e.g. Outlook does.

I was sooo sure that Horde will offer such a feature, but apparently 
not (atm). :(

Now reading your explaination gives me some hope. May I ask about the 
progress or even an ata?

Aim high...

Fetchmail is gone, not to return (for various reasons).  So that is 
not going to change.

The new code will incorporate additional IMAP/POP servers (other than 
the "master" account - the account originally authenticated to and 
hardcoded into the imp configuration files) seamlessly into the IMAP 
tree.  In other words, mailboxes on the other server will behave just 
like local mailboxes.

This issue's discription/summary is unclear to me. What will be the 
goal/outcome of the improvement.

Note that fetching email is for users in specific situations still 
wished for and needed. A new version of IMP should not cripple old 
features and option IMHO

Remove fetchmail code and allow multiple accounts to be directly 
accessible via folder lists.