The mug in the particular link that skywarrior posted works as such. I'm paraphrasing from the description given on the website (under the pictures), but it seems like you put the loose tea leaves in the middle section, pour hot water in, screw the middle section along with the top lid onto the mug and turn the whole thing upside down to "brew" the tea. Then when you are finished steeping the leaves, you turn the mug back over (so it is right side up), unscrew the middle section, set it aside, and drink your tea.
I have the 12oz. mug that is on the left hand side in the top picture. I love it! It doesn't do the 'flipping upside down thing'... err... I guess it could because it has a strainer but the 'middle section' of the mug I have is a lot smaller than the mug that was described above. So, I'm not sure if it would give the leaves enough room to expand. However, I still love it. I don't brew my tea in it but I do put already made tea in it to bring with me on the go.
I love the fact that is it clear because I get loads of questions as to what it is exactly that I am drinking. I took it with me to our Thanksgiving get together and I think from the conversations that the mug itself had started, I ended up converting some of my family members to tea.
So, heh, you can say that I have no regrets in my purchase of the mug!