1. One important piece of information missing from the help pages is
that YouTube requires the caption index number for each caption:
without that, the file is rejected. The correct format would be:

Index number
Time codes
Text
Blank line

2. Couldn't the lines be longer? Generally speaking, two lines of text
is the maximum, because it takes a significant amount of time to go
from the end of one line to the beginning of the next, especially with
centred (or, incidentally, right-justified or ltr languages) text.

3. A strange bug: If I put a comma at the end of any line except the
last, the comma is doubled, like this:

First line,
next line.

Is displayed as:

First line,,
next line.

4. The timing is slightly off. The titles appear several frames
earlier than specified.

Here are the fruits of my labours, if anyone's interested:

http://www.youtube.com/watch?v=u5B5RF3lqes