Well, this is somewhat entertaining (probably because I don't use Peep - those of you who do probably find it a lot less funny). Twitter flipped the switch on an application login change on Monday, and as a result users can't login to Twitter through HTC Peep. It's a bit difficult to understand without going into more detail, so without further ado, more detail.
Previously, applications could use one of two ways to login to Twitter: Basic Authentication and OAuth. The Twitter devs say it quite nicely, so I'll just do less work and quote them: