Yeah, i've noticed something like this too… However, i'm not sure whether it started with 10.3 or 10.3.1 - my ISP always used to be hard to dial into under Jaguar, but not as problematic as it is now
Using Jaguar, I used to have to occasionally try one or two times to get a connection (and i could tell if it was going to work or not because the scrolling 'connecting' message in the menu bar would only appear briefly if it was about to connect, but would scroll for longer if the connection was bad. Also, i'd often have to connect the phone-line to my iBook to get a connection, if it kept failing to conect with the iMac (it was as if the iBook reset whatever was wrong with the line).
Now, using Panther, it seems as if the OS is even more temperamental regarding dial-up connections. I'll be able to connect very rarely first time, but most of the time, the menu bar tells me connecting, then authenticating then disconnecting (its not that there are authentication errors - i've had this before and you get a different error message).
Normally, the only way to get a connection is to dial-up, fail to connect, wait about a minute then try again - which seems to get me a connection 2/3 of the time! (strangely, the ISP tech i spoke to tells me that his logs show we get a successful connection every-time...)
I have my suspicions that some of my problems are due to the poor line quality, and the fact that BT installed a DACS line-splitting device to save on copper wire for our neighbourhood. I've never been able to get a connection higher than 28,000 bps on this 56k modem (iMac DV 400) or on the iBook (SE 466)
Additionally, Panther sometimes seems to drop the connections before the 2-hour time-out set by our ISP. This didn't happen under Jaguar.
Our village gets broadband this Winter. We now just have to hope that the line quality is sufficient to support ADSL and that BT will actually remove the DACS box...
Anyway, sorry i'm not more helpful

, but at least you know it's not just you
