Clement, Pavano, Vazquez (the first time), Johnson, and Kevin Brown are throw-away examples.
Clement only increased by .89 runs, but pitched to a first half ERA marginally higher than the 3.68 he posted in 2004. Hindsight shows that his arm was toast.
Pavano started the year very well last year, but got shelled a few times before hitting the DL. Hindsight again shows this is injury related more than a league adjustment.
Vazquez (first time) started the year well too in '04, but it has been widely reported that he pitched through a sore shoulder in the 2nd half. His second jump to the AL only resulted in a .40 gain.
For Brown and Johnson, look at this....
Player Year League ERA
Brown 2003 NL 2.39
Brown 2004 AL 4.09
Brown 2005 AL 6.50
Johnson 2004 NL 2.60
Johnson 2005 AL 3.79
Johnson 2006 AL 5.00
They got even worse in their second year in the AL. Given their age and back problems, it's much more likely that the cause was physical than it was league adjustment.
Guestimating the number at one is complete crap. If it was 1, then the league ERAs would reflect that from year to year, but they don't. The difference is usually around .30 runs.