I've successfully (I think) imported a forum with 4,000,000+ posts in just a tad over 3 hours. I did this with a modified version of the importer script that requires shell access to the server. There were a number of changes made but the key one, as far as I can tell, was to change the definition of the empty ubbt_POSTS table to not include any of the indexes except the primary key, do the import, and then create the indexes.
Things to watch out for:
1. There is a limit to the size of MyISAM tables. There is a parameter in my.cnf that may need to be increased for tables of this size. I used: myisam_data_pointer_size = 8. (I was burned when the last 9,000 or so posts were not added because of this limitation.)
2. Adding the FULLTEXT INDEX will take a *lot* of time. For large tables if you don't have enough space for temporary files the technique used for building the index can take, literally, days. The parameters of interest are myisam_sort_buffer_size and myisam_max_extra_sort_file_size. I set both of these to something like 10 gigs. With enough space the days were reduced to 30+ minutes.
I hope this helps someone.
Chuck
