Scott Ferguson

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.

The real answer is that you should wait for JSP 2.0. (You can play with it in the Resin 3.0 snapshot. Bug reports are welcome :-))
With JSP 2.0, any RT tag attribute automatically can use a JSTL expression. You don't need to modify your tag library at all, it just automatically works.
You also get the new <jsp:attribute> capability for free with no changes needed to your tag library.

You may or may not want to use that method; that's why Resin has FastCGI support. If you look at that page, you'll see that PHP calls chdir(), which may or may not mess up your server depending on whether you use File() and FileInputStream() assuming the current directory.
The FastCGI requires you to compile PHP with something like --fast-cgi as a ./configure option. Then you also need a launching program to get it to listen to the proper port. On Unix, Resin has as bin/fastcgirunner.pl script which will launch it.
Then you can use Resin's FastCGIServlet to connect to the PHP process.
Yes, it's a little more involved to set that up. If you don't care about performance, you can also just use the CGIServlet to invoke PHP as a normal CGI.

The main JSF demos work with Resin (last time I tried.)
However, there are parts of the demo that look up files using something like new File("../webapps/foo"), i.e. using paths that assume Tomcat's file structure.
In other words, they're not even using the getRealPath() or getResourceAsStream(). It's somewhat surprising.

Resin has supported HTTP session clustering for years.
EJB session clustering will probably be in 3.0, but I'm not all that sure how much of a benefit it really is. JNDI won't be clustered (what's the point?), but there might be a JNDI <resource> which is a clustered object.