Don't know of a list for the top 50 cities, but here's Vegas.
Source:
Las Vegas Convention and Visitors Authority
2005 figures:
Visitors: 38,566,717
Hotel room inventory: 133,186 (more than any city on earth)
Average occupancy rate: 89.2%
Convention delegates: 6,166,194
Conventions held: 22,154
Domestic flight passengers: 39,316,207
Charter and international flight passengers: 2,757,593
Tourism economic impact: $36.7 Billion