There's a basic compatibility issues between Apple iCal and Microsoft Outlook/Exchange. Apple iCal strictly adhere to the iCalendar standard whereas Outlook/Exchange does not. My suspicion is that in one of your scenarios there's an Exchange server and in another there's not.
I've been dealing with a similar issue in the workplace for about 3 years now, so I know your pain. Since I'm a developer at heart, I decided to tackle the problem on my own and built a software product called Zamain for Mac to resolve the Apple iCal to Microsoft Outlook/Exchange incompatibilities. I originally built the product for myself, but soon realized that this can help others as well. Here are the major issues that Zamain for Mac resolves:
* Supports sending Apple iCal invitations from multiple Apple Mail email accounts
* Resolves the "unable to Accept" Apple iCal invitations sent to Microsoft Outlook/Exchange recipients
* Resolves incorrect time zones for Microsoft Outlook/Exchange recipients
* Prevents multiple attachments when viewing Apple iCal invitation emails in Microsoft Outlook
Although I know that this is a shameless plug for my product, I urge you to give it a shot because it will resolve your problems. You can learn more at www.zamain.com.
When I send an ICal invite from my Mac to my work PC I can open and save the calendar date, but my wife is not able to do that on her work PC also using Outlook 2007. I am using Snow Leopard on my MacBook Pro 17".