I have configured a LDAP resource with KDE 4.8.1. I can find my contacts in the address book application and in kmail when composing an e-mail (through the select-button to the right of the To: field). I also see my contacts in krunner, but "Send Mail to.." does not work. It opens a composer (kmail) instance, but the address of the contact is not added to the To: field. This works fine with contact stored in vcf files.
With Plasma 5, the Akonadi integration is currently not ready. It probably will not be for another year or so, and even then large parts will need to be rewritten. Resolving as LATER.