Does anyone know how to do this on the Pearl? I've tried to change the setting in Advanced Options > Message Services > Messaging (CMIME), but it seems to not make a difference when I actually go to compose.

I have the same problem...

Set that to the messaging service you want to be your default messaging service.

Of course, replies will always be from the account you received the message from, and if you're on a BES, it should default to Desktop when you send email to someone in your Domain.

Wirelessly posted (BlackBerry 8100: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0) BlackBerry8100/4.2.1 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/100)

Edit:
See above post!

I'm guessing you might have to do a reset, that's just my guess as it's solved a number of problems for me that were in a similar spectrum(such as custom ringtones for people).

has anyone figured this out? i've set up hosted BES but for some reason the default Sent From address when i compose msgs is still the @blackberry.net (BIS) one. i have changed it under the advanced options and done a reset.

Wirelessly posted (Mozilla/4.0 (compatible; MSIE 4.01; Windows CE; PPC; 240x320))

Right. If you are on BIS, you can change the sent from when using your name@xxx.blackerry.net account from your BIS setup on your providers web site.