Kudos to the forum at smartmobil.no for providing these tricks (multiple forum posts).

The threaded SMS feature of Windows Mobile 6.1 seems to be one of those features you either love or hate. It makes an SMS conversation resemble an IM chat between two parties. This may or may not feel “right” for you. But even if you like the concept, it may not be as smooth as you like it. For instance you may find that there is not as many lines allocated to you answering an SMS as there are lines presenting the previous dialog. And maybe you think there’s not enough room for text in general. I decided to collect some “hacks” here

Inspired by Marco’s article (http://blog.enterprisemobile.com/2008/10/writing-custom-gpos-for-scmdm-2008/) they are also provided in a GPO-friendly manner

Disabling / Enabling threaded SMS:
This is controlled by the following registry key
HKEY_CURRENT_USER\Software\Microsoft\Inbox\Settings\OEM\
DWORD SMSInboxThreadingDisabled
1 = Disabled, 0 = Enabled (default)

Specifying DPI for the current font (more equals less):
HKLM\ControlPanel\Stylus\192DPI
DWORD CurFont, Min value 600, Max value 1100
This setting is dependent on the resolution your device is running, and may not work on all devices. Most devices upgraded from WM 6.0 to WM 6.1 are running QVGA (320×240) whereas newer devices running WM 6.1 out-of-the-box run VGA (640×480). It will affect your entire inbox, meaning both SMS and mail. The text will become smaller so make sure it’s readable

And if you want it as an ADM-file:
CLASS MACHINE

CATEGORY “Windows Mobile Settings-Extended”
CATEGORY “Threaded SMS”

POLICY “Disable Threaded SMS”
KEYNAME “SOFTWARE\Policies\Microsoft\Windows Mobile Settings\Registry\HKCU\Software\Microsoft\Inbox\Settings\OEM\”
EXPLAIN “Disable the Threaded SMS feature.”
VALUENAME “SMSInboxThreadingDisabled”
VALUEON NUMERIC 1
VALUEOFF NUMERIC 0
END POLICY

POLICY “DPI”
KEYNAME “SOFTWARE\Policies\Microsoft\Windows Mobile Settings\Registry\HKLM\ControlPanel\Stylus\192DPI”
EXPLAIN “Adjusts the DPI (size) of the font for the inbox (mail & SMS). Note: is dependent on the resolution the device is running.”
PART “DPI Value” NUMERIC
VALUENAME “CurFont”
MIN 600
MAX 1100
DEFAULT 600
END PART
END POLICY

END CATEGORY
END CATEGORY

Allocating lines pr reply (sorry, this one is file-based):
Locate the file \Windows\thread_sms.html, (from a Windows Mobile 61. device), and open it up in your preferred html editor. Notepad will probably suffice for this simple file.
Find the line “Inbox:content id=”COMPOSESUBJECT” width=”98%” height=35” and change the “height”-parameter to something else. 35 is the default value in my case, changing it to 75 gives you 6 lines.

If you want to get creative there’s also an thread_style.css file to change more of the appearance.

There are probably a number of other settings available as well, but I found these to address some of the more annoying aspects of threaded SMS