The most important difference between Lotus Symphony and OpenOffice that really matters, would be the fact that Symphony is, while based on OpenOffice code, still closed-source, whereas OO is entirely open source.
Engine-wise, it should not differ too much from OO since Symphony is a derivative fork from the OO project under IBM's heading. For now, underpinnings should still be the same, however it is not too impossible to expect that Symphony will start having major internal differences as time goes by, and IBM starts deviating Symphony from OO more in order to suit their own objectives better.
You would be right in saying that for now, the main differences between Symphony and OO would be the IBM branding and different interface. However, the interface is NOT just slightly different, and actually constitutes major differences in how you work in the office suite. Two chief differences are :
- Tabbed interface in Symphony
- Sidebar for quick options access in Symphony
- Different toolbar layouts in Symphony
That said, the underlying engines are pretty similar, however Lotus Symphony might provide better integration with IBM products in the future. If you would like to find out more about Lotus Symphony, you could start off with the General FAQ here.