Fortunately that's not likely to be an issue very often, because it's only when using the 3.5mm connection that the device's own volume control needs be used. Wirelessly connected devices can take remote control of the ZiiSound D5's volume level. As such there's no dedicated remote included with the speaker - not that we ever found that a problem.
To say that the ZiiSound D5 uses Bluetooth doesn't tell the whole story; Creative is well aware that Bluetooth isn't the best transport for quality audio. As you know, the Advanced Audio Distribution Profile (A2DP) provides a maximum possible 723kbps of bandwidth which, in the real world where theoretical maximums are never reached, leads to degradation of audio transmitted via Bluetooth.
To counter this, the ZiiSound D5 uses a third-party codec called apt-X, which compresses the audio stream. The downside of this is that a dongle is required, as it has the encoder built-in. One is provided in the box for the iPhone and, in fact, any iPod with a 30-pin dock connector. Rumour has it that future Creative players will come with native support, but there's no official word on that yet.
There are benefits to the use of a dongle, though. The primary one is hinted at by the three metal contacts on the dongle's back. These match up with a set found in a slot located towards the back of the ZiiSound D5, into which an iPod can be docked. So the ZiiSound D5 is actually an iPod speaker dock in a sense.
Another advantage of using a dongle is that there's no need to pair with the speakers. If you have more than one iPod in your household that's actually quite a big bonus - getting the ZiiSound D5 to cycle through paired devices can be harder than it sounds.
The ZiiSound D5 is also compatible with the same USB dongle bundled with the Inspire S2 Wireless speakers, as they also feature apt-X. Otherwise, any device with Bluetooth built-in that's able to stream music over that connection will work, albeit without the advantage of apt-X doing its magic.