Dan,
From the Release Notes: [2006-04-27: REVISION 985] [...] [NEW] TextMate will prompt for confirmation, if you open a larger than 10 MB file from Finder.
Is your software update preference set to check for “Cutting-Edge” updates?
Yes, it is. But I opened the files via a "mate ." command which opened a project window of a folder that contained two laaarge files, clicking on one of those gave me the beach ball for a while. (as well as 200% processor load, yikes).
This is all with build 1070