The How-To (get a unpaged map)
First get the URL of the KML export. When viewing your Custom Map, right click the 'KML' link and copy the link to the clipboard (probably something like 'Copy Shortcut' or 'Copy Link Address' - depends on your browser)
Now display your KML again. I have found it best to open a new Google Maps window, probably just go-to http://maps.google.com and paste in the URL copied earlier INTO the search box, and click the Search Maps button.
In order to keep the map working at reasonable speed Google Maps needs to limit the available features visible at once, quite simply the more features shown the slower the map gets.
For "My Maps" the features are split into pages, so that a reasonable amount of data is shown on one page - effectively enough that will work at a reasonable speed. This normally results in between about 60-200 features at once, but sometimes that is not enough.
NOTE: The following paragraph may be inaccurate, Google Maps recently updated how KML files display. More results appear to be available possibly even all, but unsure at this stage
However Google Maps uses a different limited method when displaying KML files; a regional geographical culling based method. A certain number of features are shown, to give a reasonable overview, but then as you zoom in more features become visible. This often results in more results being visible, but importantly doesn't impose the arbitrary pages, so its often more friendly on the user, as can just zoom in on an area rather than flick though pages to find their area
And because Google Maps offers a KML export of a My Map, AND can display remote KML files, can exploit the two functionalities together to get a slightly better display method.
1. Viewing KMLs on Google Maps (which this trick uses) does not support the My Maps 'Snap to Road' feature, so if you use that or you Save Directions to My Maps, then the resultant map will have streight lines rather than road hugging lines.
2. 'Search Results' - Results saved to "My Maps" via the 'Save to' feature - only export the title, not the description so such results will not show on the new shinny duplicate map either.
3. There is a 3Mb limit on KML (import/export) via Google Maps. So if your map is particully big - ie lots points or description, then there is a chance your map will silently fail. No error message is shown, it just fails.
4. There is a 1,000 feature limit on a single My Maps export, and approximately 1,000 in displaying KML - so large maps wont particully benefit from this trick. Wont show an error just the first 1000 points/lines/features.
5. Saving the resultant map back to My Maps (using "Save to My Maps") is broken, and NOT recommended.
This new 'map' remains a live copy of the original My Map, so you can continue to edit the main map and this copy will automatically update (subject to a small delay due to caching)
This also means that the map can be embedded - useful to embed the original My Map but with out paging. And to the end user should barely be able to tell the difference.
 You need to click 'Link' to get a embeddable map, before you add it back to My Maps. Ie just after you have pasted in the URL into the searchbox (as per the HowTo section)