Hi!
With the help of graphviz, I have written a small perl-based tool that allows you to generate ladder diagrams. It can be found at git://git.osmocom.org/gen_ladder.git
For your reference, I'm attaching a sample input and output file.
The bent/curved arrows are a result of graphviz trying to indicate that the message is between e.g. MS and MSC and 'bypasses' BTS and BSC. I'm still waiting for somebody with more graphviz skills to make this an option.
Hope this is useful for some of you...