As you can tell from the responses, this topic is subject to some controversy. Generally speaking, the main benefits of pitching early are
- oxygen absorption
- quick pH drop (limits activity by other organisms)
The second point may be debated by folks saying "I've never had an infection from pitching late", but I think the issue is a bit more subtle. Brewing at home, all our beer is destined to be "infected" to some degree. Airborne contaminants always make it into the wort. Under the "right" conditions, their growth will be limited by hops, scarcity of oxygen (for some microbes at least), low pH and, once fermentation is well underway, ethanol. Getting healthy yeast in there is a key step here. Even if your beer won't taste obviously infected, I think there's a good chance your beer may not come out quite as clean as it could.
Now, the issue with pitching too warm is mostly the formation of esters, which happens primarily during the early stages of fermentation - so pitching warm and then cooling down is problematic. What's more, the cooling will impose stress on the yeast, which can lead to a number of problems down the line: increased production of fermentation byproducts including esters and fusel alcohols, sluggish fermentation, ...
So, yeah, we are in a bit of a pickle here! When I was living in Hamburg, there was a guy at the local homebrew club who was famous for brewing the best lagers. When I approached him about it, he said he'd pitch a small amount (like a tenth of the starter volume or so) of yeast at elevated temperature and then wait until fully cooled before pitching the rest.
I think in terms of oxygen absorption, even a small amount of yeast works (compare the "yeast oxygen scavenging method" used by some people to deaerate their strike water). The pH drop, on the other hand... Not quite as much, probably, it'd be interesting to get some measurements here.
As far as ester production goes, I was sceptical: aren't we essentially doing a massive underpitch at an elevated temperature? Mister Lager's take on it was that the small amount of yeast wasn't able to get anything going within the relatively short timespan. I can't really judge whether that line of reasoning is accurate, but his Franconian lagers convinced me that he must be doing things right.