(update Dec 22. : Version 3.6 removes this problem!)
I had changed nearly 20 Thousand photo-files while migrating from IMatch to Lightroom 3.
Changes werde made to IPTC-Record’s field: caption / abstract.
On trying to import those changed files to thumbnails already in LR’s catalog with synchronize folder or by read metadata LR crashed.
Tracking down the problem I found, if the import time was short because I had tried only a small folder, there was no crash.
Knowing that the automatic write of XMP changes typically starts after a 15 seconds delay after a change, I tried to change the setting:
Menu / Edit / Catalog Settings / Metadata / ‘Automatically write changes into XMP’ from yes to no.
Now the import / sync even for all folders at once succeeded.
As far as I can see, all content is ok.
Update: Bad news
When I now set the option to ‘Automatically write changes into XMP’ LR crashes immediately!