Not sure, I've only used it for my own personal development stuff, plus some small projects I'm working on, but I have not seen it anywhere in "production". I tried to search from some J2EE hosting sites that actually used Resin as the container, but couldn't find any (well, other than the one Frank had many moons ago). Not sure why that is, I figured there at least had to be some shop out there offering it.
Matthew has it right. Resin doesn't directly compete with Websphere and WebLogic (except in engineering terms). The market segments from a business perspective are very different. The sorts of companies who will willingly pay $20,000 per CPU and buy million dollar machines are very different from the $500/server market. WebLogic/Websphere can't sell into the $500/server market and Caucho doesn't want to play the political games for the $20,000/CPU market. The small business market isn't sexy and it's never going to excite the press, but it's a solid business. In terms of product future, Resin is planning to implement everything in J2EE 1.4, but continue to concentrate on the servlet/JSP/JSTL/XML/JSF end of things. So from a technical end, it is competing with WebLogic/Websphere, but less so from the business side.