Will encrypting my Droid X2's SD card make me lose the ability to view files on it from my PC?
According to this forum post, the SD card is not fully encrypted.  Rather, after you enable encryption any new data is encrypted.

So for example, if you take a picture, then enable encryption, then take another picture, the first picture will be unencrypted but the second will be encrypted (assuming your pictures are saved to the SD card).  If you pull out your SD card and try to look at the pictures elsewhere you'll only be able to view picture #1.

However, putting your phone into PC mode and connecting it to a PC will allow you to view and copy both the encrypted and unecrypted files.  At least according to that post.  So to answer your question: No, you will not lose the ability to plug in your phone to your laptop and edit or view data on the SD card.