Although I have had my BlackBerry PlayBook for almost a year now, I only recently got around to buying a micro HDMI cable so I could hook it up to a television. Long story short, it is a lot of fun (and will be even more useful once we get the Bridge remote later this month).
On Twitter Jeff Bacon mentioned that it should just be included in the box, and he is totally right. I bought the cable online for $2.99 including shipping. Once your remove the extra shipping costs, the (surprisingly large) amount of packaging, and buy it in quantities somewhat larger then one, you could probably assume that RIM could package this with the PlayBook for less then a dollar.
Why would RIM want to do this? Well, because it is a pretty cool feature and adds a lot of out-of-the-box functionality. Also because it is something that neither the iPad nor the Amazon Fire can do. Once OS 2.0 and the Bridge remote is available, this will be even more cool.
If it takes an enthusiast like myself nine months to get around to picking up a cable, you can assume that most costumers probably never will, even if it only takes one use ever for this $3 purchase to pay for itself. At the very least it is more useful then the micro USB cable that is included. I already have plenty of USB cables, and never plug my PlayBook into my computer anyhow.
Until RIM starts including it, I recommend that everyone with a PlayBook to spend the few dollars and buy a cable. It is well worth it.