Just confirming - Small Business Server 2008 is still going to be a 75-user product.
There's been some confusion lately (our fault entirely) due to a brochure that said SBS is good for businesses with up to 50 PCs. This was merely some marketing copy that referenced the sweet-spot & should have called out that the physical limitation was still 75 (though very few businesses actually run it at that level).
Essential Business Server will also support up to 300 users, but it remains to be seen what the sweet spot will be. Given that its most likely incarnation (in the early days at least) will be as an upgrade path for SBSers, I personally think it'll be between 50 - 150.
Source:→ Microsoft Australia SBS blog
Microsoft, SBS, Small Business Server