[dartho]

This will modify the post text and append a message to it, this means that the user could then edit the post again and remove the text if they wished, but it works for the most part. Disabling the pplugins will not remove the text from posts already made.

Create 2 plugins and change the XXX to the styleid number of your mobile style:
1.
Product:vBulletin
Hook Location:newreply_post_start
Title: Posted via Mobile Device NewPost
Execution Order:5
Plugin PHP Code:

if ($styleid==XXX) {
$vbulletin->GPC['message'] = $vbulletin->GPC['message'] . "\n". "Posted via Mobile Device" . "\n";
}

2.
Product:vBulletin
Hook Location:newthread_post_start
Title: Posted via Mobile Device NewThread
Execution Order:5
Plugin PHP Code:

if ($styleid==XXX) {
$vbulletin->GPC['message'] = $vbulletin->GPC['message'] . "\n". "Posted via Mobile Device" . "\n";
}