When I download Mac Transmission 2.3 from CNET and Transmission the DMG says: The Following Disk Image Cannot Be Opened: Transmission-2.30-dmg not recognised.
I had the same problem. I have two Mac's and the download verified on only one of them. Based on what I can tell, the signature for the new version was probably generated with a newer version of openssl (OpenSSL 1.0.0d 8 Feb 2011). The version of openssl that OS X uses (OpenSSL 0.9.8l 5 Nov 2009) is older. If you have the newer version available, you can verify the DMG now. Otherwise, we will have to hope the developers see this and create a new signature and repost the DMG.
I went back to CNET and they have fixed the .DMG. And it was a bad signature, because when I tried the auto update it said that it failed do to a bad signature. Problem is el solved