unfortunately, I haven’t found a solution yet.
I get the ("unscientific") impression, that things improved slightly with the dev build 2220; something definitely got changed compared to build 2217 (the 2.0.1 release version), since in 2220 you see tiny dotted lines, something like a cursor contour, when pressing alt-u. However, while I was hoping this meant things got fixed, the situation remains essentially unchanged, as of yet. After a couple of minutes, every time, all the time, alt-u stops working.
Maybe (but again, this is an unscientific observation), the problem is triggered even faster when using full screen distraction free mode (which I think is very nice).
As of now, this alt-u problem is a show-stopper for me, going back to TextMate for the time being -- and missing sorely ST2’s distraction free mode and its overall nicer code display. (And speed.)
I’m going to try the USGermanKeyboard layout (thanks very much for the link!), this sounds interesting. Hopefully though, nothing else is changed besides the Umlaute input. In every other respect I very much prefer the US layout. (I post back when I have tried it out.)
Here’s hoping that ST2’s alt-u issue is fixed over time.
NB.: Maybe I should note that I am on a 10.6.8 32-bit system, i.e. on a rather dated one. I’m getting hit with problems on other occasions too, since developers are by now often unable to check their code on 32-bit systems. So maybe you should check the situation on a "modern" 64-bit Mac (all models since late 2006, i.e. since "forever"), maybe the issue doesn’t exist there.