I just finally worked this out and thought others may find it useful someday as Google failed me....
I couldn't get lightbox working due to silverstripe's customised prototype.js (the conclusion I came to anyway) so I used thickbox
PHP Code required:
//Block prototype
Requirements::block('jsparty/prototype.js');
Requirements::javascript('jsparty/jquery/jquery.js');
// DON"T load jquery_improvements
// Requirements::javascript('jsparty/jquery/jquery_improvements.js');
// Load thickbox
Requirements::javascript('mysite/javascript/thickbox/thickbox.js');
Requirements::css('mysite/css/thickbox.css');
Prior to this I was running into errors such as $(domChunk) is null which in IE is Error: 'null' is null or not an object on Line: 22 or "$ is not a function"
It is all about $ used by both prototype and jquery. Unfortunately jQuery.noConflict() doesn't seem to work with ThickBox.
Sources:
http://silverstripe.org/installing-silverstripe/show/254725?start=0
http://ssorg.bigbird.silverstripe.com/archive/show/137145
http://open.silverstripe.com/ticket/3941
It should be possible to use the built in greybox (or whatever it is). Feel free to share your experiences with modal windows.