A quick Google search will produce this information. Furthermore, I think a survey of new server deployments would be even more in Ubuntu's favour, though gathering such information is no doubt difficult.
In the article it does mention that Ubuntu is the most popular distro for Amazon deployments...
The article cites Cloud Market's data on AMIs, which indicate how many AMIs have been derived from the Ubuntu base AMIs, rather than actual deployment numbers. It's an interesting indicator...
Having been on the inside (EC2) and seen the numbers, I can confidently say that Ubuntu's AMIs are the most popular AMIs deployed on EC2. The stock AMIS take the top spots by an overwhelming majority, and most of the lower spots are filled with derivatives of the original Ubuntu AMIs.
Ubuntu's been working hard in the server/cloud space for a while, between the JeOS effort and subsequent, excellent tools for building customized installations around it. It's not just a brand or that makes them popular; with Ubuntu you get a sane system, style, and toolset for your system images.
Some anecdotal evidence - our university is running free Linux VM's for research purposes (so, non-graphical remote servers), people can choose what they want, and the majority seem to choose Ubuntu; with CentOS being second place.
Source?