WPMU vs WP 3.0 MultiSites

Looking into some difficulties of upgrading to WMPU 2.9.1 I cam across an article about a merge of WP and WPMU.

Some say do not build in WPMU and wait for WP 3.0.

I need some guidance on this. I am building what is to be a major foundation of our business on WPMU. We will be moving over 15,000 blogs hosted.

What is WP 3.0 and what are the compatibility issues between WMPU and 3.0?

Any immediate feedback would be greatly appreciated.