Been experimenting with minecarts, but haven't quite figured them out yet. Can't get my powered minecart to push me at any reasonable speed. I thought it said it pushes you at 8m/s (twice the walking speed), but I'm barely hitting like 3m/s. Ah well, more practice.
Also, I have a toooon of wheat from my two tiny farms. And even with widening my greenhouse in two directions two blocks, I still have only been able to get one tree to grow at a time. I guess I'll just keep messing around with the arrangements of torches and trees.
I don't know how you're going about building the tunnel but the technique I use is to make a 2 wide by 2 tall (depending on how wide and tall I want the tunnel) stretch of dirt along the floor of the lake. The dirt displaces the water. Then I line the dirt with glass or whatever material I plan to use. Then I just go in from one end and dig out the dirt.
On the subject, I just completed this yesterday.
http://img.photobucket.com/albums/v662/Lemunde/Minecraft/2011-04-29_142758.jpg
http://img.photobucket.com/albums/v662/Lemunde/Minecraft/2011-04-29_142805.jpg
http://img.photobucket.com/albums/v662/Lemunde/Minecraft/2011-04-29_142807.jpg