Ok, I've looked at Kirby's conversion script, and it looks fairly straightforward. One could almost assume that a bit of copy/pasting and changing a few things would allow to convert the attachments as well. Hmmm.
On second look, my non-programmer's eyes find potential holes, big enough for a backhoe to fall in

The attachment's table in SMF seems to require much more info than that of SB.
SMF:
CREATE TABLE {$db_prefix}attachments (
ID_ATTACH int(11) unsigned NOT NULL auto_increment,
ID_MSG int(10) unsigned NOT NULL default '0',
ID_MEMBER int(10) unsigned NOT NULL default '0',
filename tinytext NOT NULL default '',
size int(10) unsigned NOT NULL default '0',
downloads mediumint(8) unsigned NOT NULL default '0',
PRIMARY KEY (ID_ATTACH),
UNIQUE ID_MEMBER (ID_MEMBER, ID_ATTACH),
KEY ID_MSG (ID_MSG)
) TYPE=MyISAM;
SB:
CREATE TABLE `mos_sb_attachments` (
`mesid` int(11) NOT NULL default '0',
`filelocation` text NOT NULL,
KEY `mesid` (`mesid`)
) TYPE=MyISAM;
Comparing both, it looks like it might be possible to do it relatively simply? Before I start trying things out and killing my DB's (backup backup backup), could you confirm my thoughts are on the right path?
Using the basic structure of kirby's file, I would add another step.
In that step, I would convert SB's mesid to SMF's ID_MSG
I would then convert SB's filelocation to SMF's filename
I would need to somehow do a query to retrieve the associated member name in SB's messages tables so it could be inserted in SMF's ID_MEMBER. This bit I don't know how to do at this point. I don't see it

Caveat: filelocation is much different data than filename. in SB's table, it contains the full path to the file, and the filename (i.e. '/home/username/public_html//components/com_simpleboard/uploaded/images/Stress Symbol.gif') So I guess on top of fancy query work to retrieve username, there needs to be some fancy footwork to strip all the path except the actual name, right? How would one do that? Any ideas?
Caveat: would leaving size as 0 cause problems?
Thoughts and assistance on this would be *greatly* appreciated.