A fewof those who came over to WordPress from MovableType recently have faced problems with importing huge MT export files (we are talking about import.txt files the size of an mp3 file – approx 5MB). The problem is that the server runs out of memory allocated for php processes when processing so large a file. Server admins usually set the memory limit to a reasonably low value, especially on shared servers, to prevent resource hogging.
One solution for the problem where the MT importer times out before completing its task, is to break up your import.txt file into smaller files. Another is to use a custom template to export entries by the month, or year, and then import those files into WordPress.
This Library Planet article has tips, and MT templates to try both these methods.
For more help regarding moving from MT to WordPress, refer to the moving guide.