More than 280 million active accounts -- see the press release for more details. I've been using the beta for several months and consider it a much better user experience than, e.g., the Gmail client.
Microsoft Corp. today announced that Windows Live™ Hotmail®, the successor to MSN® Hotmail, is launching globally in 36 languages. The most significant upgrade for Hotmail since it pioneered the webmail industry in 1996, the new service has been built to be a vast improvement over the previous Hotmail offering, having incorporated input from more than 20 million beta testers. Windows Live Hotmail will deliver a safer, more powerful and productive e-mail experience than previous versions with flexible access via the Web, on a mobile phone or with an e-mail client. Microsoft also announced that later this month Windows Live Hotmail customers will be able to access their Windows Live Hotmail e-mail and contacts for free* using Microsoft® Office Outlook® 2003 or Office Outlook 2007 via the new Microsoft Office Outlook Connector beta.