getting from Interlaken to Venice

You don't need to get a reservation for trains in Switzerland.
You do need a resevation for Inter-City and High Speed trains in Italy.

You will need to buy a ticket from the Swiss border (probably Dommodssola if you are coming from Interlaken), to Venice. This train does need a reservation. When you buy this ticket (on http://www.trenitalia.com/ the Italian railways website) it will come with the reserved seat automatically. When you get on the train in Switzerland (at Spiez or Brig), just go to the seat reserved for you from the border.

P.S. www.bahn.de is The German railways website. They only sell tickets within, to or from Germany.
Otherwise your plan sounds OK as long as you plan enough train travel to cover the pass, but I assume you have already done that calculation.

THANK YOU!!
Can I ask another question? Should I purchase the Swiss Travel Pass and the ticket you mentioned above NOW before we leave the US? Most of the sites I read say you just walk up to a train and hop on - no problem. But if I need to buy a reserved ticket, should I go ahead and do that or can I purchase when we arrive in Switzerland.

And since I will have the Swiss Travel Pass, how do I KNOW that the city you mentioned will be the connecting city in Italy? How do I find that out so that I am buying the correct reserved ticket?

Devans,
You can buy the Swiss Travel Pass in advance on Swiss Federal Railways website ( www.sbb.ch ), or when you arrive at Zürich airport station. The price is the same, your choice.
You should buy Italian tickets as soon as possible, because the longer you leave it the more it will cost. But you can only buy 2-3 months in advance.

"But if I need to buy a reserved ticket" - but you don't reserve rail tickets in Switzerland. It is a turn-up-and-go service. Just turn up at the station, look up the next train to your destination, get on, find an empty seat and enjoy the ride.
The exception is the 3 big tourist special trains (Glacier Express, Bernina Express and Golden Pass Express). These are extra trains with panoramic windows (there are normal trains on the same route), and in this case you reserve seats in the panorama cars.

How to find your route? Go to the abovementioned Swiss Federal Railways website ( www.sbb.ch )
Enter from Interlaken Ost, to Venezia S. Lucia, enter a date and time.
Choose a connection (for example the one at 13:29 with only 2 changes). Click on '+' to get the details.
Then click on one of the train numbers ("EC") to see all the stops for that train.

Look at the Swiss Travel Pass Validity map here: https://www.swisspasses.com/railpass/overviewmap_en.pdf
This shows you the Swiss pass is valid up to Domodossola or Chiasso, depending on which route you take.

YOU ARE AWESOME!
And you live in Switzerland which makes me happy! Which means I didn't get an answer from someone in California! (No offense, California!) Thank you very very much!

Ok since I've gotten you to advise me on train travel, would you give me your insight on: YES DO THIS and NO DON'T DO THAT? We will be staying at Hotel Rossli in Interlaken. Right now, our itinerary looks like this:

Day 1 - (following Rick Steve's recommendation) -
train to Murren
cable car to Schilthorn
hike down to Gimmelwald orLauterbrunnen
train to hotel

Day 2 -
train to First Cliff Walk at Grindelwald-First
cable car to Mannlichen
hike to Kleine Scheidegg
hike to Wengen
train to hotel

Day 3 -
Trummelbach Falls OR boat on lake OR train to Lucerne for our final day

To be honest, this will probably be my once-in-a-lifetime to see Switzerland. I want to experience the beauty of the mountains. I don't have to do every touristy thing on the map.

If "this will probably be my once-in-a-lifetime to see Switzerland." don't stay in Interlaken. Stay in Murren, or at the very least in Lauterbrunnen. Interlaken is a transportation hub and tourist trap. You will have a much more memorable experience if you stay in the mountains.

I never understand why people choose to stay in Interlaken and then take transportation into the mountains. If you want to see the mountains why not stay IN/ON the mountains where the views are amazing! But, then.....I'm from California so what would I know.

Susan/Monte - I think it's rather obvious that people who stay in Interlaken are probably placed there by travel agents who've always heard "Interlaken" being thrown around in Swiss Vacation conversations. Anybody who's actually every been "thru" Interlaken and on into the Lauterbrunnen Valley and Murren/Wengen etc KNOW why we don't stay in Interlaken.

Much the same reason people come here asking questions about Eurail Passes...

Yes, true Tim, and I realize that. You don't know what you don't know. Until you have been somewhere you can't imagine it or know what you are really getting.

Maybe there is a draw to stay in Interlaken that I'm unaware of? It is a beautiful lake, so maybe the people that stay there want a lake experience not really an alp experience, or they want a bigger town experience? Or, like you said, they think it is the "Swiss alps" but will find out when they get there that it isn't.

I highly doubt tourists come to Interlaken to see the lake...although the lakes are indeed beautiful. They come to see the alps. Little do they know they're so close but not quite there. To step foot outside your hotel door and be in the middle of the alps is breathtaking...and no more expensive than trinkety souvineer-laden Interlaken.