We (http://pivotalsf.com) have a client/project that uses an f5 load
balancer to spread load across three web/app servers running apache
2.2 backed by a mongrel cluster (mysql and solr on separate hosts).
This works great. Our only (minor) problems with this have been with
the lb's config properly detecting 503s from apache. I highly
recommend apache for this.
Hope this helps,