Even though IE8 has gone under extensive development already, to the point that even Microsoft has suggested people use it over IE7, the company is still not ready to release a final version this year. At the very least, Microsoft has one more public beta in the works, which is assumed to be the precursor to the first IE8 Release Candidate.
The first RC, though, isn't expected until next year – with no specific release dates available yet. If they push it much into the first quarter of 2009, it would mean that IE8 spent a year in beta. Microsoft has stated that they'll be focusing on “critical” issues from this point on, being hesitant to add any more functionality to the browser. What also hasn't been discussed yet is whether or not Microsoft will make IE8 a mandatory update or attempt to push it out as they did with IE7.