Fan page Duck and Cover has an interview with Black Isle Studios' Feargus Urquhart online, talking about Fallout 3. The game hasn't actually been formally announced yet, but it has been repeatedly confirmed, and while Feargus doesn't provide too much in the way of new information, he does explain what the delay has been in getting full-scale production the project off the ground, as noted in this excerpt:

We did start Fallout 3 in early 1999, but things were not moving forward that quickly. Plus, we needed to get another project out relatively soon because of Interplay's financial situation. So, the Fallout 3 team became the Icewind Dale team. We had then hoped to start Fallout 3 after Icewind Dale was completed, however at that time both Chris Parker and I had to jump in and spend pretty much every waking moment on Baldur's Gate 2 until it shipped in late September of 2000. Since we were going to be very busy and not able to help a Fallout 3 get started, I decided to have the team work on the Heart of Winter expansion. Since the team completed that in February of this year, they have been working on the miscellaneous projects - but have started thinking about Fallout 3. Plus, a lot of people have been transferred over to work on TORN to help that get done as quickly as possible.