Is an android encrypted SDcard readable/decryptable easily by a desktop machine? If I encrypt my SDCard on my android phone using the built-in encryption software, and my phone dies for whatever reason, would I be able to easily by just using the pin/password to access that data later on my desktops [OS X, Windows and Linux]?
The Android developers page on Full Disk Encryption seems like it has all the info you want, and more! It talks about encryption on the main internal disk (/data), I will assume that it uses the same mechanism for encrypting the external SD Card.

In a nutshell:

When your turn Android Disk Encryption on, it generates an AES-128 key which it uses to encrypt the drive (called the master key). It then takes your Pattern / PIN / Password and uses that to derive a second AES-128 key which it uses to encrypt and store the master key on disk. You know how the first thing Android does during boot-up is ask for your PIN / Password? That's because it needs it to decrypt the master key so that it can decrypt and mount the main filesystem. The reason for having two AES-128 keys is that if you change your Pattern / PIN / Password, it just has to re-encrypt the master key file, not the whole disk.

Is it decryptable on a desktop?

Theoretically, yes.

The location of the master key file is no secret, so if you know the PIN / Password, and you know which key stretching function Android uses to turn this into an AES-128 key then you should be able to decrypt the master key, and then dercrypt the disk. Whether there is freely-available desktop software to do this, or you would have to write your own, I have no idea.

(One possible hitch is that I don't know if it stores a copy of the master key file on the external SD Card, or if there is just one copy on the internal flash memory chip. Finding a way to access the internal flash memory on the dead phone might prove to be the hard part here. You could probably check pretty easily if there's a key file on the encrypted SD Card by popping it into your laptop and looking.)