HtmlSelect value not available when using callback instead of post

[Zoodor]

I have a server control that contains an HtmlSelect. It currently uses
postbacks to function (and works fine), but I am upgrading it to be able to
use callbacks (using postback or callback, depending on a property that the
page developer can set).

I have two instances of the control on my page, one set to use postbacks and
the other to use callbacks. When I pick a value from the select in the
postback control, the value of the select in BOTH controls is available in
the postback handling code (which is fine) but when I pick a value from the
select in the callback control, only the value from the select on the
POSTBACK control is available in the callback handling code (in
this.Page.Request.Params).

Can anyone explain what's going on here and how I can get the select in the
callback control to have its value posted back for callbacks? I'm probably
being really stupid, but its got me kinda stumped.

Thanks