Well, at Prominic.NET we have been doing Domino hosting for about 12 years, now.  It's our core business and we have offerings from small shared accounts up through Enterprise-grade configurations, and we'd be happy to work with you.  You can find us, naturally, at http://Prominic.NET

 

If you want to compare and contrast a bit, though, Domino Developer Network, Connectria, and CoreFusion are a few other companies who also offer Lotus hosting and should have this size of package.