To summarize, the GHWT drums are the easist solution, but lack of standalone availability makes the option somewhat painful.  Buying and building the MSA-P is highly flexible, but requires a lot of hacking and is time consuming.  The adapter from sethmeisterg works very well, but not working with the XBox 360 is going to cut out many of the potential userbase.  Finally, the J5 Kickbox takes a different approach from the other adapters by accepting individual pad inputs, but the lack of drum brain controls is cumbersome and potentially game affecting in some cases.

The MSA-P from Highly Liquid

The first practical, somewhat affordable, solution for connecting a drum kit to Rock Band, and the basis of a large portion of this FAQ.  Up to now, building this box has been the only option for connecting a drum kit to the XBox 360.  Because of the encrpytion used on the XBox, an existing Rock Band drum kit must be used as an intermediary part of the solution.  It is fairly complicated and time consuming to build this adapter, although obviously this FAQ simplifies the process as much as it can.

This adapter is not capable of velocity sensitive output…although, to be fair, the velocity sensitive features of both Guitar Hero and Rock Band drumming have been rather widely panned.

If you have a choice between the MSA-P and the GHWT controller, keep in mind that the MSA-P is soldered directly to the wired kit and consequently has about 5-10 ms less lag than any wireless solution.  If you are an extremely serious player, this sort of difference is noticeable and can affect your scores.  I always switch to my MSA-P when playing Rock Band, even though I have a hacked up GHWT box for Guitar Hero as well.

MIDI adapter from sethmeisterg

This home brewed adapter produced by a forum member on rockband.com serves as an equivalent to the MSA-P converter, except it acts a direct converter.  While somewhat clunky looking in appearance and suffering from an occasional bug or two with updates to the Rock Band platform, many PS2 and PS3 owners with this box seem to be quite pleased with it and have even gained new functionality over time such as velocity sensitivity and cymbal detection.

There is no version of this adapter for the XBox 360 due to the controller encryption on the platform.

J5 Kickbox

This is actually an adapter that accepts the 1/4" TRS jacks of electronic drum pads and triggers.

While slightly easier to deal with than the MSA-P, this approach eliminates some of the advantages of involving the drum brain in the solution, such as managing crosstalk and triggering between pads.  It also makes switching between the drum brain and playing Rock Band very cumbersome.  Personally, I would skip this.

Guitar Hero: World Tour drums

The drum kit for Guitar Hero: World Tour is likely to be the best option for a converter for any platform now that it is available.  The GHWT drums have a MIDI input .. what else is there to say?  You just need to make sure you map your MIDI output notes to what the GHWT drums expect … this is listed below.  Also, the controller section is fairly easily separable from the rest of the kit with some hacking, making it a nice and modular piece that can be mounted to your own electronic kit.

The biggest current disadvantage is that you can't find these drums to purchase standalone yet as of 1/1/2009.  Obviously, this will change at some point, but we don't know when.

In summary, as a mass produced solution that is easily connected to any electronic kit, modular, and capable of being used on all platforms, most of the home brewed solutions from the community are likely to be obsolete moving forward.

From http://www.scorehero.com/forum/viewtopic.php?t=80389

Red=38
Yellow=46
Blue=48
Orange=49 (unused in Rock Band)
Green=45
Purple=36 (orange in Rock Band)