Unless you've managed to nail a topic that absolutely nobody else is podcasting about, you should consider who they are, what they're doing, and how you can use them.
Who Are They?
It's not uncommon for a person who is already well known in an area to start podcasting about their area. It will be hard to compete against a famous person providing the same content that you intend to provide and that may be reason enough to pick another space.
On the flip side, pretty much every famous person has detractors out there that don't like them. You may be able to grab that portion of the potential audience that like the subject matter, but don't like the individual.
What Are They Doing?
There are many ways to provide content and someone else working in your space may be producing content in different ways than you intend. Business tips, for example, can be provided by series of interviews by one podcaster and by a series of experiements and book reviews by another. You can probably co-exist nicely with other podcasters if you're both doing things differently.
Can You Work With Them?
A show that is already well established in your space might well turn out to be an ally rather than a competitor. The GNU/Linux space that I operate in is a great example of this. There is a handful of us producing podcasts about Linux, but rather than compete with each other we use each other's content, guest on each other's show and generally help promote each other.