I'm not quite ready to say it's the Holy Grail of closed captioning, but Subtitle Studio for Windows is really pretty good. It is not buggy, like Magpie, and it saves to any of about 20 different formats, including SAMI.

It can be downloaded from HERE.

The formats supported are:
DVDSubtitle
IAuthor
JacoSUB 2.7+
MacSUB
MicroDVD
Phoenix Japanimation Society
PowerDivX
RealTime
SAMI Captioning
SonicDVD
SubRip
SubSonic
SubStation Alpha
SubViewer 1.0
SubViewer 2.0
TMPlayer
Turbo Titler
ZeroG