Just in case you felt as though Windows Phone didn't already have enough mobile messenger apps available on the store, Threema is looking to release an official client. According to the website's frequently asked questions (FAQ) section, a Windows Phone app is actively in development, but with no release date published. If you're not familiar with the service, it's a popular end-to-end encrypted messenger available cross-platform.
All that's required to use the app after downloading it (once it's available for Windows Phone) is to link your e-mail address and/or mobile phone number so other people can find you. You can synchronize contacts from that point and begin exchanging secure messages. What's cool about Threema is there are verification levels for contacts, to ensure you're actually communicating with who you believe you are communicating with.
When you synchronize your contacts, the public keys will be retrieved from the server, which has verified that they belong to the respective person via SMS or e-mail validation. This is the "orange" verification level. However, they recommend you don't trust them and verify the keys by yourself the next time you meet your contacts in person. Threema lets you easily do that by scanning a QR code (which contains the ID and public key) directly from the mobile phone of another user. Once scanned, the verification level will change to "green".
We're not sure how recent (or old) this FAW entry is, but it's good to know they're at least looking at Windows Phone. Head to the Threema website to see for yourself. Scroll down to the "General" section of answers and see the other platform support enquiry.
Source: Threema; thanks, Mark, for the tip!