EDIT 2014-02-07: Eclipse Luna is here, and support for Tomcat 8 is included in the bundled WTP : ) Happy days!
Tomcat 8 is still in development, but
I have tried below and it worked for me.
To add the Tomcat 9.0 (Tomcat build from the trunk) as a server in Eclipse.
Update the ServerInfo.properties file properties as below.
server.info=Apache Tomcat/@VERSION@
server.number=@VERSION_NUMBER@
server.built=@VERSION_BUILT@
server.info=Apache Tomcat/7.0.57
server.number=7.0.57.0
server.built=Nov 3 2014 08:39:16 UTC
Build the tomcat server from trunk and add the server as tomcat7 instance in Eclipse.
ServerInfo.properties file location : \tomcat\java\org\apache\catalina\util\ServerInfo.properties
The latest version of Springsource STS (3.6) supports Tomcat 8. It is based on eclipse Luna 4.4 and supports Java 8. Have at it!
Alternatively we can use eclipse update site (Help -> Install New Features -> Add Site (urls below) -> Select desired Features).
For Luna: http://download.eclipse.org/webtools/repository/luna
For Kepler: http://download.eclipse.org/webtools/repository/kepler
For Helios: http://download.eclipse.org/webtools/repository/helios
For older version: http://download.eclipse.org/webtools/updates/
In addition to @Jason's answer I had to do a bit more to get my app to run.