Find an open source .NET project on Codeplex or Sourceforge that interests you and join, contribute. Build up a few months of a track record of making contributions to that project, collect URLs for file change sets you've checked in (depending on the source control system: for CVS this is possible), and provide those to a prospective employer to check out your work.
Identify one or two highly focused newsgroups or forums you can follow (avoid small fry sites, go to big ones like silverlight.net or www.asp.net). Read a lot first, then try contributing there, answering other .NET developers' questions. Collect URLs of threads that you've helped solve, and after a few months of a track record, provide those to a prospective employer to check out your level of knowledge and trouble-shooting.
If you're a good writer and can commit to keeping it up, Blog. Blog about the .NET technology, and what you've learned, and ideas about applying it that you have. Post sample code that you've worked on. Don't spout off about non-.NET on your Blog, if you have political opinions, express those on a separate blog using an alias. Give your Blog URL to prospective employers so they can see how much you know and how passionate you are about .NET.
Volunteer to help somebody set-up a web site, small business owner or non-profit or charity. Opportunities here are infrequent, but some sites like volunteermatch occassionally are looking for people who have the skills to set-up or enhance their web site. Call the volunteer coord. and ask questions, many of these sites will be Apache-run or LAMP and not ideal for you. While you are volunteering, network with anyone you're collaborating with to find out if they know of any career opportunities. When you're finished, if everything went well, ask for a letter of reference that you can give to a prospective employer so they can see what a great soul you are for helping the Children's Hospital spruce up their Web site.
It may sound like I'm suggesting you do a lot of free work for others. 
But the common thread is: do positive things with the technology, and make sure you build a public track record of your accomplishments so you can present that to prospective employers. I regret this won't work overnight -- unless you are exceptionally lucky and network with the right person -- but it will eventually work out.
- Derek