You are correct, that tape is used to hold the cables in place. Especially with an HTC phone, they have a tendency to run lots of cables everywhere! The tape that is being used is called Kapton Tape. Its a heat resistant thin tape, and has multiple uses in the electronic repair industry. iFixit sells this tape for about $10. You can also find it on eBay for about $5 (both links below). You could probably apply a little electrical tape to hold the cables down, I wouldn't use too much since electrical tape is thicker than the Kapton and you don't want that thickness to interfere with something not laying properly.
[product|IF145-113-2]
http://www.ebay.com/itm/10mm-X-100Ft-Gold-High-Temperature-Heat-Resistant-Kapton-Tape-Polyimide-BGA-/111313114679?hash=item19eac75637