The details of the plea bargain were basically: plead guilty, get a psych eval, and presuming they don't find him to be "mentally deficient" or a danger to society, be on probation for some amount of time and pay some unspecified fine. He pled (which you might have been bored enough to read; the details of which would seem to preculde getting a fair trial later) on (8/8/77), went to Chino for an up-to-90-day eval, released after 42 days; probation report (9/17/77[*]) indicated no interest in supporting jail time, nor that he was considered a deviate or a danger to society.
[*] http://www.thesmokinggun.com/archive/years/2008/1203081roman28.html
The appearance that he skipped town for was the final one where the sentence would be confirmed; everyone thought and wanted it to be this way (family, prosecutor, defense), and the judge gave a strong signal in the day or two before it that he was going to surprise everyone with additional jail time. The one piece of the recomendation that didn't happen immediately was financial restitution, which happened later as a result of a settlement of a civil suit.
Here's a good summary:
http://www.latimes.com/news/local/la-me-polanski-legal1-2009oct01,0,1100452.story
<clip>
The men said in the film and reiterated in court declarations
this year that Rittenband improperly sent Polanski to Chino
for the purpose of punishment rather than testing. The judge,
they said, had agreed to set Polanski free after that stay,
but reneged and decided to imprison the director again at his
official sentencing in what amounted to a second round of punishment.
"I told Judge Rittenband that the diagnostic study was not designed
to be used as a sentence, but [he] said . . . he was going to do it
anyway," Gunson wrote in an August declaration in appellate court.
</clip>
It makes Dennis laugh, but it seams reasonable enough to me. The impact of his exile is nearly the same (if not worse) than actual deportation, which was one of the threats. It also tends to discredit the idea of him withdrawing his plea after all the other things had already happened.
/jordan