David,
We are using XBAPs with IE, but not disconnected (so can't help you there)... We started off with Silverlight, but then for a few reasons moved to WPF:
1) We could use functionality in custom .NET libraries we already have.
2) We want to embedd the maps in other windows applications, and have bi-directional communication between them, so we can just as easily compile as a stande alone app too.
3) Plus having full .NET availalbe is nice :).
But other than a few layout/graphical glitches that were fixed by tweaking the controls template... they work great! Also found a few issues with touch screens, for example with the legend control, the scroll bars intercept the touch and do not pass down to its child controls (like a slider bar), and this issue does not exist in SL.
But all in all, works good for us. But we started when SL3 was out, and as SL keeps getting more robust with every version, we might have to switch one day.
Anyway, hope that helps a bit, sorry I can't comment on working disconnected.