Still happening, too bad Mac users are such a small consumer percentage
This has been happening to me since the 2nd of July Updates of L4D2 and the Portal 2 update today.
I would like to mention I have had a lot of trouble with steam issues in the past, from validating cache to the in-game community and the most annoying of all is that my games despite being set up to auto update seem unwilling to do so and there is no mechanism to force an update. But nothing has been quite as consistently annoying as this one bug.
The initial problem was that L4D2 would not launch past the title screen, it would just crash. This then evolved into the server too busy error.
These are all the things I have attempted to fix the problem:
First off I did the usual "verify game cache", this would sit at 25% and then either skip to the end and say confirmed, or it would say servers are too busy to perform this function, OR steam would crash.
Obviously I tried changing download regions, I've tried all different servers for my region along with various nearby regions.
I've tried deleting the steam app (but not the app support files "steamapps") and reinstalling it.
I attempted renaming the clientregistryblob and restarting steam.
I tried deleting the .ncf (no cache files) for L4D2 (this is a suggested fix on steam support); this merely caused L4D2 to attempt to reinstall, it got about 5% in then froze and now it just sits on "download starting"
I tried deleting all L4D2 content and restarting steam, same problem as above.
This problem seems to be getting worse, I am now permanently in offline mode when I log into steam. I've also noticed that it has been progressive over the last month or so, my usual download rate for games is between 300kb/s and 1.5mb/s but in steam for the last few game updates it's been sitting at 17kb/s with a max of 35kb/s (and this is in three separate houses with 3 different ISPs all with at least 100gb dl limit). This is also true of my sister and my flatmates macs, both of which also seem to be getting the "server too busy" error today. Suck that this seems to be a mac issue only (all of my friends who have run steam today on windows have had no problems) as we are such a small percentage we are unlikely to get any patches fast.
My two cents, let me know if anyone finds a proper fix.
Steam severs too busy are happening to me, too, in Windows and OSX. It's probably related to their big Summer Sale and other people accessing the game installation/updating servers. This is not uncommon when a big name title is first released or these big sales are ongoing. My advice is to keep trying, you'll get through as soon as their servers have room for you.