The mobile phone I travel with is pretty good and I use JabberMixClient to instant message from it. Version 2.1 finally got rid of the avatar-replacement bug, so I only wanted to solve my inability to make an SSL connection to our servers. (The linked email is slightly wrong: it’s signed by our own Certificate Authority, not self-signed.)
The answer was to install TTLLP’s Certificate to the phone, as suggested on Sony Ericsson’s developer site. After a few unsuccessful atttempts, I corrected my two mistakes: firstly, I needed to install our CA certificate, not the server certificate; secondly, use PEM format, like on tjworld’s k800i. The exact command was
obexftp -b $PHONE -p ca.pem and then the phone prompted me whether I wanted to install it.