using getfield to split up URIs

You suppose wrong, well almost...

Everything can be a variable so you can dynamically create a pool name based on inbound variables and then use the pool command (or with any other command)

Your rule doesn't work as-is but it would with a slight tweak. "$cust_id_pool" would be evaluated as the variable which doesn't exist. What you'll want to do is enclose the variable "cust_id" in braces like this:

when HTTP_REQUEST {
  set cust_id [getfield [HTTP::uri] "/" 4]]
  pool ${cust_id}_pool
}

There are examples in the forums where customers are mapping portions of the hostnames and sections of the uri's into pool names, using portions of the hostnames to redirect to another uri on a common hostname for use in vanity domains,etc. You are really only limited by your imagination in what you can do.

I've been trying to highlight iRules that kind of stand out in my blog. You might want to take a peek when you get a chance:

http://devcentral.f5.com/weblogs/joe/category/28.aspx

Click here

Also, if you aren't already, click the "subscribe" checkbox at the top of each forum to get email notifications on each post. It's a good way to get some good ideas as well.

Oh, and hop on over to the TCL reference at Sourceforge for any language specific issues (such as regular expressions, binary scanning, string manipulation, etc):

http://tmml.sourceforge.net/doc/tcl/

Click here

Good Luck!

-Joe