Although it isn't often recognized, there is a distinction between "headset" and "headphones". The difference is usually branding.
A headset is often used for communication. They usually have lower quality speakers (to reduce price), and have a microphone.
Headphones and for listening only, and don't typically have a microphone, but will include speakers that are often proportional in quality to the amount you pay for them.
That being said, there are devices that try to fill both roles. This is why most people tend to get them confused. The line between "headset" and "headphones" can be rather blurred; however, when talking about bluetooth devices, this distinction becomes very important.
Bluetooth uses different protocols to communicate, depending on the type of data being transmitted. The three most common protocols for this application are:
AD2P - A uni-directional protocol, used for streaming 2 channels of high bandwidth data (audio). This is typically used for music, but will not work for headsets because the data can only travel one way at a time.
HSP - This is the primary bluetooth audio protocol, and is supported by nearly every bluetooth "headset". It is full duplex, meaning audio can travel to a from the unit at the same time, at a cost of bandwidth. As such, the downside with this protocol is that the sound quality tends to be lower. This protocol is not supported yet by the A500 (or any android tablet, AFAIK). If you get a device that requires this protocol, it will not work.
HFP - This is for hands-free features, and can work in conjuction with either of the above protocols. It allows you to control your call/music features, such as smart-dialing, volume or track skipping.
If you are looking for a "headset" that works with the A500, you might be out of luck. Based on your post, I'm guessing you're looking for "headphones", so I'll give you my recommendation, I personally like the RF-MAB2 by rocketfish. The sound quality is great in AD2P mode, and they work with my cellphone as a headset. They do support HSP and HFP, but as mentioned above, the A500 does not support the HSP protocol. The skip/forward/volume/etc buttons will work though. I like the behind the head design, which allows me to wear a bike helmet, or a hard hat (I work in construction) while I wear them. I was so impressed, a bought a second set. They usually retail about $40.
Hope that helps.