I had the same issue as the OP: Win 7 (x64), TortoiseSVN (x64), and DropBox (x86). The info from some of the other answers gave me all the info. I've only ever had the x64 version of TSVN installed on this machine.
In my case TSVN and DropBox were installed the same day I did the OS install and the overlays worked fine until a couple of days ago. I did nothing involving changing settings for either app to cause them to stop working.
Here is what I had in the icon overlay registry section after the problem started (HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\ShellIconOverlayIdentifiers
):
I verified that only the overlays corresponding to the first 11 entries display in Explorer. When I modified the order of above entries by adding 'z' to the start of some of them, again only the first 11 overlays (under the updated order) would display.
With the above I had everything I needed to solve the problem (either rename or or delete entries so that the TSVN entries I want working are <= #11 on the list). Below deals with wondering why this suddenly happened.
I know that based on the overlays that worked prior to a couple of days ago, keys 1-3, 7-9, 12-13 were all <= 11 in the list (not sure if overlay #14 ever worked since I never had files w/ read-only status. #15 never worked on this machine so i know it was never in the top 11). I also assume the block of TSVN keys move up/down in unison, therefore they were bumped down either two or three places (* see below). This implies that 2-3 items were added between the DropBox & TSVN blocks. The three that are there now are added by Windows and I would assume they'd be there as soon as the OS installed.
Is the list of 15 overlays determined at run-time? Seems like the overlay handlers might sometimes tell the windows shell that there are no icons to add to the list. Possibly some settings I messed with a couple of days ago related to file sharing and file encryption caused some of those items at the 4-6 spots to become "activated" and push the SVN ones down.
In the end I deleted a couple of entries and moved some, so my final list looks like this: