Recent Outlook versions can synchronize with Horde via ActiveSync/EAS.

You know it would be really great if Horde could be updated from 
Outlook via WebDAV. I appreciate that Outlook may not be publishing it 
correctly, but is there any chance someone could swallow their pride 
and help all those Outlook users out there who would love to use their 
Horde calendar instead but can't keep the two in synch?

Stalling unless/until someone can reproduce this with current H5 code.

Outlook 2007 shipped with WebDAV support.  Go to the All Folders view, 
right click on Calendar, "Publish to Internet", "Publish to WebDAV 
Server."  It'll even do it automatically when you make updates to your 
calendar.
Horde itself states "500 Internal Server Error" in its own logs.   
Around line 157 of "/var/www/html/horde-prod/lib/Horde/RPC/webdav.php".
Well, that's the problem I'm trying to work around - people can get 
Outlook uploading to iPHP Calendar via WebDAV and to Apache 2 WebDAV 
folders, so while the problem is Outlook, I don't think the 
solution/workaround lies in Outlook.
Yes, it is wrong, which is why I've been trying to fix it with Apache 
rewrite rules to be correct. (renaming username/My_Calendar.ics to 
username/username.ics using a regex)  However, Horde doesn't seem to 
pick up on the rewrite, even though the rewrite is definitely applied 
according to Apache logs. (perhaps Horde is relying on what Outlook is 
sending as the URL?  Outlook doesn't follow redirects for WebDAV, only 
rewrites)

If Outlook gets fixed at any point, we can rely on many users not updating. :)
On my Windows XP VM, the user-agent is pretty distinct: Microsoft 
Office/12.0 (Windows NT 5.1; Microsoft Office Outlook 12.0.6425; Pro)
I'm just wondering if Outlook isn't sending anything, possibly because 
of the feedback it's getting from Horde when Outlook does its PROPFIND.

Since when does Outlook support WebDAV? How did you setup Outlook?
Apache is probably returning that error, not Horde. Did you check the 
web server logs?
Yeah, great. :) How is this supposed to work if Outlook is sending no 
content, and that to wrong address?
Of course, because it's wrong.
Ugly, but could be a workaround.
Even uglier. What if Outlook at any point in time gets fixed suddenly? 
What the user agent of Outlook anyway? And finally, how would that fix 
Outlook not sending any content?

Outlook 2007 WebDAV doesn't work, as detailed in my mailing list post here:



http://lists.horde.org/archives/horde/Week-of-Mon-20090615/037725.html



The upshot is that Horde returns a 500 Internal Server Error with no 
useful debug and Outlook 2007 likewise displays a "Server error" 
dialogue box.



Continuing my debugging I've found a couple of interesting things of 
note.  I'd post tcpdumps of the data exchange, but I do not wish to 
expose my password since I had SSL disabled to get a clean dump.   
Here's what I found out:



In the PUT function of lib/Horde/webdav.php, Horde sees these as the 
values of the important variables when hit by Outlook:

path: kronolith/username/username_Calendar.ics

content_type: application/octet-stream

content: [null]



By contrast, Lightning and other working clients:

path: kronolith/username/username.ics

content_type: text/calendar

content: [iCal file]



The path that ends in x_Calendar.ics seems unaffected by URL rewrite 
rules in Apache.



I've got a couple of clues/hunches at this point to get it working.   
Keep in mind this is simply to get Outlook working by any means 
possible, it may not be desirable for Horde to adopt these measures:

1. If the user-agent is Outlook, preg_replace $path s/_Calendar.ics$/.ics/

2. Hard-code an entry into $list in webdav.php's _list function to 
return a username_Calendar.ics entry