i've downloaded the newest drupal 6 appliance this week. but php 5.3 causes a lot of problems with most of the modules i use.
how can i downgrade to php5.2
The most reliable way would be to compile php5.2 from source, but that will also mean that you will need to take care of your own security updates manually (and will most likely mean a recompile every time).
The other option is to use another repo which has php5.2 (such as Karmic). In some repects this may be an easier solution but there are potential pitfalls. I would suggest you do a bit of research, google will give you plenty of leads and Ubuntu forums are indespensible. (FYI TKL v11.x is based on Ubuntu Lucid/10.04).
I support this ... D6 has quite a few modules that don't play nice with PHP 5.3
I suggest the appliance for D6 should use 5.2 (blocked from upgrade, I saw a technique for doing this and will post it if I can find it again) and the appliance for D7 should use 5.3
Probably the most reliable option is to use the Legacy TKL Drupal 6 appliance. TKL legacy images are based on Ubuntu Hardy/8.04 which is still supported by Cannonical with security updates to packages from the main and security repos until April 2013.
Generally any solution to use php 5.2 with Ubuntu 10.04 (the basis of TKL v11.x) will involve some sort of hack. All of these require tradeoffs that mean reduction in end user reliability and/or security and/or maintenance complexity. That is fine if you are willing to personally take responsibilty for it, but as TKL is aimed at a really broad audience (from newbs that may not understand the consquences and associated responsibility), some could argue that it would be negligent for TKL devs to produce appliances with these sort of compromises.
OTOH one of the greatest things about Linux IMO is the freedom to do what you want, so if you have a hack/solution to use php 5.2 in TKL v11.x (Ubuntu 10.04) then please feel free to post it so others can follow your lead.
This should be worked into the latest Drupal 6 release:
More information about formatting options