I have been having the same issue as you. I emailed HTC informing them of the API change. While they replied back to me within the hour of me submitting the email, they didn't help at all. Here is what they emailed me to me:
Thank you for this information. We have attempted to reproduce this issue on one of our test devices and are not experiencing any difficulty in using the Peep application. You may try clearing the application data to see if this may resolve the issue. Home > Menu > Settings > Applications > Manage Applications > Peep > Clear Data After clearing this data, please perform a soft reset by removing the battery for several seconds, then putting it back in and powering the device on. If you are still experiencing this issue, you may consider performing a factory reset. Please be aware that this reset will remove any personal data from your device, so please confirm that this information is backed up, either to your Gmail account or by some other means, prior to performing this reset. To hard reset your EVO: Home > Menu > Settings > SD Card and Phone Storage > Factory Data Reset If you continue to have this issue, please try using the Twitter application as mentioned previously. We will continue to look into this issue to determine if we can duplicate the results you are receiving.
I cleared the cache and did the battery pull. It didn't work. I do not have any desire to do another factory reset. I just did one a couple weeks ago after the Froyo release. My phone is running fine, other than Peep is no longer working.
So my solution was, I went and got Twitdroyd Pro. I had downloaded the free version the weekend before the Froyo release and was trying it out. I liked it, but I ultimately went back to Peep because I liked the UI (I love me some Sense). But now it looks like I'll be with Twitdroyd for a while. Its working right now, and has other options that Peep doesn't. I would suggest the free version. The Pro version has good widgets and skins. The Pro skins really improve the UI.