You don't need to use the Keyword window to add keywords in iPhoto 08. If you type a new keyword into the keyword field under a photo (make sure View » Keywords is set), it will be automatically added to the list of keywords (and becomes available in the pop-down list of guesses that shows up as you start typing in the keyword field).

So, by setting your view options to show keywords alone (not titles, etc.), you can Tab quickly between the keyword fields of a group of photos, adding new keywords as necessary, thus getting essentially the functionality of KeywordAssistant. (The only thing missing is the warning that you are adding a new keyword.)

[robg adds: Keywords are greatly improved in the new iPhoto (unlike, say, photocasts, which were summarily dropped). After you've created some keywords, you can make them even easier to use by creating quick entry keywords. Open the Keywords window (Window » Show Keywords), then drag your often-used keywords up into the Quick Group area. Once you've done that, you can assign keywords when browsing by simply typing the highlighted letter for the appropriate keyword in the Quick Group area. If the keyword has already been assigned to the photo, then typing its letter again will remove it. And yes, this behavior is documented, but just in case you haven't flipped through the manual yet...]