WordPress 2.8 should be released any day now (it was supposed to be released in April, but I am fine with waiting a few more days). Frequently there are some changes from version to version which affect plugin developers. I decided to try out the development version a few weeks ago, and found that they had drastically changed the widgets API. If you were using the suggested functions such as register_sidebar_widget instead of wp_register_sidebar_widget, you should be fine. However, under the old API, the only way to get multiple instances of a widget was to use the wp_register_sidebar_widget function, which I have been using in my plugins for the past year or so.
WordPress 2.8 has a new Widget API which creates a generic widget class. It is now much easier to get multiple instances of widgets, though it did take a fair amount of code rewriting for me. If you are starting a new widget plugin though, you will definitely be happy (especially once it is well documented). I figured out the new API by looking at the default-widgets.php file, and also reading a few tutorials here and here.
I now have alpha versions of each of my collapsing plugins available. If you are already testing out WP 2.8, or when you upgrade, please check them out. To download them, visit the appropriate plugin page on wordpress.org, and click on the “other versions” link, then try out the alpha version. They are alpha, so there is likely to be bugs. I appreciate your patience.
Finally, I have to say that wordpress 2.8 has some very nice features besides the new widget API. Above all, it is much faster, both on the administrator side and on the blog side, mostly due to better loading of javascript.