I understand why browsers are moving towards this, but I must say I don't care for it. I especially don't care for IE 9's buggy attempt .
Try searching for ASP.NET using one of these combined address/search bars in any browser. Yep, you wind up onhttp://asp.net instead of doing a search. Most of these implementations allow you to explicitly indicate you're searching rather than navigating. For instance, in Chrome you can type "search: asp.net" to do a search. However, don't realize that what you typed "looks" like an address and you'll have to try again with this alternate search syntax, which is frustrating. I could probably learn to live with it if I have to, but IE 9 has to be fixed first.
Type "search: asp.net" in IE 9 and it will try and run something on your computer. What, I don't know, I just know you get a confirmation dialog asking if it can run something.
You can put the text in quotes, which will do a search, but quoting also changes the behavior of the search and won't always be what you want to do, so I hope that's not the only answer that IE 9 is going to have here. At a minimum, following everyone else and using the "search:" syntax is almost mandatory. It would be nice if you could use the name of an installed search provider, allowing one to "bing: asp.net" and "google: asp.net" without changing the current search provider.
So far this has mostly been about a design flaw, but I claimed IE 9 was buggy. Here's the bug. Try to search for ASP.NET MVC. In Chrome, this just works, no special syntax required. In IE 9 and attempt is made to navigate tohttp://asp.net%20mvc. Really? That's obviously not a valid address. I personally think it's a little dubious for the address bar to be translating spaces to %20 under any circumstances, but this situation is just crazy. Certainly something to fix before IE 9 makes it out of beta.