Tomcat 7 is also at the core of Mulesoft's TCAT Server product. TCAT Server is a suite of administration controls, mostly all available in DIY configurations but neatly packaged by Mulesoft (along with significant value add), for clustering and deploying Tomcat 7 in enterprise, production environments.
While Tomcat is widely known as the "reference implementation" of the JSP and Servlet specifications, it has long been used in production, mission critical, environments. IMHO you can scale as noted by Tanuj both ways and very reliably. However, how much you want to build versus buy in this regard may lead you to products like TCAT Server.
Timothy Stone, MIT, SCJP
"This Satan's drink [coffee] is so delicious, we shall cheat Satan and baptize it." --Pope Clement the VIII (1592-1605)