ArcPad does do related tables though it is a round about way to get to them. Double click your form for your feature class in ArcPad Studio (on the tree for your layer, it will be labeled <FORMS>), highlight the feature class in the "Forms" window, and the button "Related Tables" becomes active, select that button. In the "Related Table Forms" select your related table and select the button "Forms". From here you can build your form for your related table.
The other way to open the related table form is to expand your Layer tree, it will be under FORMS-EDITFORMS-TABLEFORM. Double click TABLEFORM and your related record form should open up.
Hope this is what you were looking for and it helps.
Bryson
