you know that you can buy some templates for as little as 5 bucks I've actually created something like 20 sites just playing around here for IM with google sites and use those as part of my portfolio and also wp has a multiple sites feature were you only do 1 installation and you can have several sites.
You can offer to do some simple sites here and add those to your portfolio or go on craigs list and offer to do some free sites "the domain and hosting
not included" and add that also, some one was offering on a diferent forum to do free logos just so they could increase the portfolio, there are many many ways to go about it just be a bit creative.
Regards
Suave
PS I must add it does not matter that the client knows that you are using WP most of them don't know or don't have the time to do it them self,
example:
I got a buddy of mine from college that does some driving over the weekend (limousine) at first he told his boss that I would create a site for the company and have it live then he went and purchase the domains from goddady and figure out that he could just create the site him self told the owner that I did the work and pocket the money not its been 4 months the site looks like shit, I am sure that my bud (he told me he read a SEO book so he knows all about it)

has no idea what he is doing and he keeps calling me to fix the shit mess that he did, payback is a bitch at first I figure I do him a favor and charge him 99 bucks for creating the site, hosting and support and a couple of citations now I told him that e will have to pay min of 499US per month, he mention he is going to ask his boss for the cash im sure he is going to look for some else to do that wait till he starts getting quotes for 1100-1800 just to get the site fixed
