There is a huge change with widgets code in WP2.8 (amongst many other code changes), and unless your theme author says the theme is specifically compatible up to 2.8, you may have to wait awhile. 2.8 is a pretty big release in terms of needing code updates to keep individual themes working properly.
From personal experience with the theme I've been developing, singular widgets are likely to be fine for now, but any multi-widgets will probably need some rewriting to be compatible with 2.8. I was using the same code that makes up the new 2.8 multi-widgets for my theme prior to 2.8, and I still had to rewrite a lot of code to make the multi-widgets 2.8-compatible. Any multi-widgets coded with just the WP documentation as a guide are going to need a lot more work than what I had to do.
Dynamic sidebars in general have gotten some updates as well, and things that used to work fine pre-2.8 might not be compatible with 2.8+
If you value your theme over the new widgets and theme administration, you might not want to upgrade to 2.8 until the theme author announces that the theme is compatible.