The problem is not if WP can handle 1 million post, the problem is if your server can handle it.
I have a autoblog farm of about 200 autoblogs with resource intensive plugins -- to handle this I have a dedicated server with 16GB EEC RAM and a fast quad core processor, i have 3 hard drives on it. One is the main hard drive where the apps are installed on, the other is dedicated to back ups and then the 3rd is dedicated to MYSQL.
The hard drive dedicated to MYSQL is a fast 150 gb 10 000rpm drive ( If i had money i wll get a faster solid state). Wordpress is a database driven software as such i believe being able to access the database file at max speed is crucial, hence th reason i have a dedicated fast hard drive for this.
Getting to the info fast is one problem solved, you also need a fast CPU to process all the info and then RAM is also super important, the more the better.
If you going to expect allot of traffic u will also need a fast dedicated line on your server, i have a dedicated 100MBit line .
Hope this is aof any help