What version of Matlab are you running? The example runs fine on my computer, R2007b (Mapping Toolbox 2.4). But the line numbers you list don't agree with either of my copies of the code (the inpolygons line you note as 59 is line 69 in the code offered here, and the line you note in polysplit doesn't appear at all in my version of polysplit.m).
22 May 2007
Got an error just running the example.
??? Error using ==> vertcat
All rows in the bracketed expression must have the same
number of columns.
Error in ==> polysplit at 35
if ~isnan( lat(length(lat)) ); lat = [lat; NaN]; end
Error in ==> inpolygons at 59
[xsplit, ysplit] = polysplit(xv, yv);
01 Mar 2007
Works perfectly and very quickly!
The Mapping Toolbox saves me again!