I agree with the previous posts about disabling WP caching whilst working on the site, but I would like to add that the Yahoo (and a number of other large hosting providers) employ dynamic content caching systems globally load balanced, -- english translation, they store you're php output as "html" in a caching appliance at various sites globally in order to reduce overall network traffic for duplicate requests, you might also consider "pragma-no-cache" headers, in order to avoid the caching servers/appliances completely, at least while working on the site.
One of the "quirks" of most of the caching systems used by the Yahoo's/Aol's, etc of the world is that they generally work very well with things like the "timestamp" or time modified on html files, and so will reload them almost immediately upon seeing timestamp changes, ironicaly, dynamic content from php files get caught in a "loophole" in that system, so it can take some time before the cache "reloads" the content. css files are treated as html files with regards to time modified, so will reload almost immediately upon changes.
Almost all large hosting providers AND ISP's are using some type of caching or other these days (I'm on a comcast cable modem, and they use caching extensively, fortunately (for me), it ignores most dynamic content), AOL is about the worst, yahoo is probably somewhere in between.
I use CI-HOST for hosting a couple of dedicated servers and co-location, I've been with them for around 6 years, and they're really great. They too, "optimize" thier network with caching however. So, my point is, that it's almost impossible today to avoid having your content cached somewhere (especially images).
For sites where I'm either doing lots of work, or I want to ensure users are getting fresh content, I use some/all of the following -
<META HTTP-EQUIV="PRAGMA" CONTENT="NO-CACHE"> and/or
<META HTTP-EQUIV="CACHE-CONTROL" CONTENT="NO-CACHE"> and/or
CONTENT="Mon, 22 Jul 2002 11:12:01 GMT">
So, as I mentioned, while wp "caching" could and likely is part of the issue, the various caching servers also have an impact as well. It's worth noting, that if you'll be using any of the meta tags, the "pragma" tag from HTTP/1.0 has more or less been depreciated by the "cache-control" tag in HTTP/1.1, so for most caching servers that would be the more effective and preffered method of controling how/if your content is handled.
Part of the issue with the caching servers at the moment, is that your php generated content is handled differently than a "known" file type like .jpg, .htm, .css, etc. The caching servers basically store your php generated content as something like
I have some previous experiance with caching and load balancing, and this all really became apparent to me in the past year or so as I had a number of AOL users contacting me saying, I made a post and keep hitting "refresh" or "reload" but I my post doesn't show up for hours.
If anyone needs/wants any further detail on the caching server process employed by the web hosting/isp companies, feel free to contact me at chradil at comcast dot net with your questions.