If i subscribe to a weekly podcast will the last one automatically be lost from my ipod?

The default settings in iTunes is to keep all episodes for a Podcast. However you can adjust the Podcast Settings in iTunes depending on what you want.

To change the default settings in iTunes -> go to Podcasts -> then click on the Settings button (at the bottom) -> a Podcast Settings box will pop-up -> Then use the "Episodes to keep" drop down list box to change how many Podcasts you want to keep.
Choose from:
- All episodes
- All unplayed episodes
- Most recent episode
- Last 2 episodes
- Last 3 episodes
- Last 4 episodes
- Last 5 episodes
- Last 10 episodes

I will typically set it to keep the "Last 3 episodes", so that I don't run out of space on my iPhone by keep lot's of old podcasts.